Set DropDownList selected value on PostBack in ASP.Net Core MVC

Hello friends,

I am developing an ASP NET CORE solution with C # and SQL Server.

The problem I have is that I require that when the page loads, the elements of the two lists are selected, with default values ​​for each one previously by the user.

I already get the value from the database and pass it to the view through the ViewBag.ClaveProdServ and assign it to the IdPredeterminado variable

For the dropdowlistFor "SAT Clave", I fill it with the variable Claves of type List <SelectListItem> ();

With the value of the variable, DefaultID, how can I select the item from the list?

@model Modelo.Facturacion.Productos

@{
    Layout = "_FormLayout";
    ViewData["Title"] = "Agregar Producto/Servicio";
    ViewData["Modulo"] = "Producto";
    ViewBag.IncluyeUrlEnForm = true;
    IEnumerable<SelectListItem> unidades = new List<SelectListItem>();
    IEnumerable<SelectListItem> claves = new List<SelectListItem>();
    IEnumerable<SelectListItem> impuestos = new List<SelectListItem>();
    string Existe = null;
    int IdPredeterminado = 0;

    if (ViewBag.unidades != null)
    {
        unidades = (List<SelectListItem>)ViewBag.unidades;
    }
    if (ViewBag.ClaveProdServ != null)
    {
        IdPredeterminado = ViewBag.ClaveProdServ;
    }

    if (ViewBag.Claves != null)
    {
        claves = (List<SelectListItem>)ViewBag.Claves;
    }
    if (ViewBag.Existe != null)
    {
        Existe = ViewBag.Existe;
    }

    if (ViewBag.Impuestos != null)
    {
        impuestos = ViewBag.Impuestos;
    }
}

<div class="row">
    <div class="col-md-4">
        <form asp-action="Registrar">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
            <input hidden value="1" id="ID_USUARIOS" name="ID_USUARIOS" />
           </div>
            @if (Existe == "SI")
            {
                <font color="red" size=4>Ya existe un producto con el mismo nombre</font>
            }


            <div class="form-group">

                <font color="red" size=5>*</font>
                <label asp-for="Nombre" class="control-label"></label>
                <input asp-for="Nombre" class="form-control" maxlength="1000" />
                <span asp-validation-for="Nombre" class="text-danger"></span>
            </div>
            <div class="form-group">
                <font color="red" size=5>*</font> <label asp-for="Clave" class="control-label">Clave</label>
                @* @Html.DropDownListFor(u => u.Clave, claves, "-- Seleccione Clave Producto/Servicio --", new { @class = "form-control" })*@
                <input asp-for="Clave" class="form-control" maxlength="10" />
                <span asp-validation-for="Clave" class="text-danger"></span>
            </div>
            @*@*<div class="form-group">
            <font color="red" size=5>*</font> <label asp-for="IdSATProductoServicio" class="control-label">Clave SAT</label>
            @Html.DropDownListFor(u => u.Clave, claves, "-- Seleccione Clave Producto/Servicio --", new { @class = "form-control" })
            <span asp-validation-for="IdSATProductoServicio" class="text-danger"></span>
        </div>*@

            <div class="form-group">
                <font color="red" size=5>*</font> <label asp-for="IdSATProductoServicio" class="control-label">Clave SAT</label>
                @Html.DropDownListFor(u => u.IdSATProductoServicio, claves,  new { @class = "form-control" })
                <span asp-validation-for="IdSATProductoServicio" class="text-danger"></span>
            </div>
            <div class="form-group">
                <font color="red" size=5>*</font><label asp-for="UnidadMedida" class="control-label"></label>
                <input asp-for="UnidadMedida" class="form-control" maxlength="20" />
                <span asp-validation-for="UnidadMedida" class="text-danger"></span>
            </div>
            <div class="form-group">
                <font color="red" size=5>*</font> <label asp-for="IdSATUnidadMedida" class="control-label"></label>
                @Html.DropDownListFor(u => u.IdSATUnidadMedida, unidades, new { @class = "form-control" })
                <span asp-validation-for="IdSATUnidadMedida" class="text-danger"></span>
            </div>
            <div class="form-group">
                <font color="red" size=5>*</font> <label asp-for="PrecioUnitario" class="control-label"></label>
                <input type="number" value="0" asp-for="PrecioUnitario" class="form-control " />
                <span asp-validation-for="PrecioUnitario" class="text-danger"></span>
            </div>
            <font color="red" size=5>*</font><font color="black" size=3>=Dato Requerido</font>
            <br />
            <hr style="height:1px;border:none;color:#333;background-color:#333;" />
            <font color="black" size=3>Impuestos</font>
            <hr style="height:1px;border:none;color:#333;background-color:#333;" />
            @foreach (var item in impuestos)
            {
                <br />
                <tr>

                    <td>
                        @{
                            var nombre = "Impuesto"; /*+ @item.Value;*/
                        }
                        @*@Html.CheckBox(item.Text)*@
                        <input name=@nombre type="checkbox" value="@item.Value" /> @item.Text<br />
                        @*<input type="checkbox" id="chk" value="second_checkbox"> <label for="cbox2">Este es mi segundo checkbox</label>*@

                        @*<label>
                        @item.Text
                    </label>*@

                    </td>

                </tr>
            }
        </form>
    </div>
</div>
@section Styles {
    <link rel="stylesheet" href="~/lib/select2/css/select2.css" />
}
@section Scripts {
    <script type="text/javascript" src="~/lib/select2/js/select2.js"></script>
    <script>
        $(document).ready(function () {
            $('select').select2();
        });
        var form = Vue.component('test', {
            template: '#test',
            data: {
                Email: 'sada'
            },
            methods: {
                validate: function () {
                    var self = this;
                    self.$validator.validateAll().then(function (resolve) {
                        if (resolve) {

                        }
                    })
                }
            }
        });
    </script>
}

My Class

namespace Modelo.Catalogos.Sat
{
    [Table("SATProductoServicio")]
    public class SATProductoServicio
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int ID_PRODUCTO_SERVICIO { get; set; }

        [Column(TypeName = "varchar(9)")]
        [Required]
        public String CLAVE { get; set; }

        [Column(TypeName = "varchar(150)")]
        [Required]
        public String DESCRIPCION { get; set; }

        [Required]
        [Column(TypeName = "datetime")]
        public DateTime FECHA_INICIO_VIGENCIA { get; set; }

        [Column(TypeName = "datetime")]
        public DateTime? FECHA_FIN_VIGENCIA { get; set; }

        [Column(TypeName = "varchar(10)")]
        [Required]
        public String INCLUIR_IVA_TRASLADADO { get; set; }

        [Column(TypeName = "varchar(9)")]
        [Required]
        public String INCLUIR_IEPS_TRASLADADO { get; set; }

        [Column(TypeName = "varchar(2)")]
        public String COMPLEMENTO_QUE_DEBE_INCLUIR { get; set; }

    }
}