Hi micah,

Refer below sample.

UserControl

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="UserControl.ascx.cs" Inherits="UserControl" %>
<table>
    <tr>
        <td>Item
        </td>
        <td>Price
        </td>
        <td>Quantity</td>
    </tr>
    <tr>
        <td>
            <asp:TextBox ID="txtItem" runat="server" />
        </td>
        <td>
            <asp:TextBox ID="txtPrice" runat="server" />
        </td>
        <td>
            <asp:TextBox ID="txtQuantity" runat="server" />
        </td>
    </tr>
</table>

HTML

<asp:ScriptManager ID="sm1" runat="server" />
<asp:UpdatePanel ID="up1" runat="server">
    <ContentTemplate>
        <asp:ListBox ID="lbCategories" CssClass="form-control dual_select" runat="server" multiple=""
            AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged"></asp:ListBox>
        <hr />
        <div>
            <table>
                <tr>
                    <td>
                        <asp:PlaceHolder ID="ph1" runat="server" />
                        <br />
                        <asp:Button ID="btnAdd" runat="server" Text="Add" />
                    </td>
                </tr>
            </table>
        </div>
    </ContentTemplate>
</asp:UpdatePanel>
<br />
<asp:Literal ID="ltlCount" runat="server" Text="0" Visible="false" />
<asp:Literal ID="ltlRemoved" runat="server" Visible="false" />
<br />
<br />
<asp:GridView runat="server" ID="gvInsertedRecords" />

Namespaces

using System.Configuration;
using System.Data.SqlClient;
using System.Data;

Code

protected void Page_Load(object sender, System.EventArgs e)
{
    if (!this.IsPostBack)
    {
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT TOP 4 CategoryID,CategoryName FROM Categories"))
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection = con;
                con.Open();
                lbCategories.DataSource = cmd.ExecuteReader();
                lbCategories.DataValueField = "CategoryID";
                lbCategories.DataTextField = "CategoryName";
                lbCategories.DataBind();
                con.Close();
            }
        }
    }
    AddAndRemoveDynamicControls();
}

private void AddAndRemoveDynamicControls()
{ 
    Control c = GetPostBackControl(Page);
    if ((c != null))
    {
        if (c.ID.ToString() == "btnAdd")
        {
            ltlCount.Text = (Convert.ToInt16(ltlCount.Text) + 1).ToString();
        }
    }
    ph1.Controls.Clear();
    int ControlID = 0;
    for (int i = 0; i <= (Convert.ToInt16(ltlCount.Text) - 1); i++)
    {
        UserControl DynamicUserControl = (UserControl)LoadControl("UserControl.ascx");
        while (InDeletedList("uc" + ControlID) == true)
        {
            ControlID += 1;
        }
        DynamicUserControl.ID = "uc" + ControlID;
        ph1.Controls.Add(DynamicUserControl);
        ControlID += 1;
    }
}

private bool InDeletedList(string ControlID)
{
    string[] DeletedList = ltlRemoved.Text.Split('|');
    for (int i = 0; i <= DeletedList.GetLength(0) - 1; i++)
    {
        if (ControlID.ToLower() == DeletedList[i].ToLower())
        {
            return true;
        }
    }
    return false;
}

public Control GetPostBackControl(Page page)
{
    Control control = null;

    string ctrlname = page.Request.Params.Get("__EVENTTARGET");
    if ((ctrlname != null) & ctrlname != string.Empty)
    {
        control = page.FindControl(ctrlname);
    }
    else
    {
        foreach (string ctl in page.Request.Form)
        {
            Control c = page.FindControl(ctl);
            if (c is System.Web.UI.WebControls.Button)
            {
                control = c;
                break;
            }
        }
    }
    return control;
}

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    int index = lbCategories.SelectedIndex;
    for (int i = 0; i < ph1.Controls.Count; i++)
    {
        for (int j = 0; j < ph1.Controls[i].Controls.Count; j++)
        {
            if (ph1.Controls[i].Controls[j].GetType() == typeof(TextBox))
            {
                if (j == 1)
                {
                    TextBox txt = ph1.Controls[i].Controls[j] as TextBox;
                    if (string.IsNullOrEmpty(txt.Text))
                    {
                        txt.Text = lbCategories.SelectedItem.Text;
                        return;
                    }
                }
            }
        }
    }
}

Screenshot