Hi micah,

Check this example. Now please take its reference and correct your code.

Database

I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<asp:TextBox ID="TextBox1" runat="server" placeholder="Insert Itemno"></asp:TextBox>
<asp:DropDownList ID="ddlCountries" runat="server" multiple="multiple"
    CssClass="multiselect" data-placeholder="Select Country">
</asp:DropDownList>
<asp:HiddenField ID="hfSelected" runat="server" />
<asp:Button Text="Filter" runat="server" CssClass="btn btn-info" OnClick="Filter" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.13/js/bootstrap-multiselect.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.13/css/bootstrap-multiselect.css" />
<script type="text/javascript">
    $(function () {
        $('[id*=ddlCountries]').multiselect({ includeSelectAllOption: true });
        $('[id*=ddlCountries]').on('change', function () {
            var selectedText = [];
            $("[id*=ddlCountries] option:selected").each(function (index, item) {
                selectedText.push(item.text);
            });
            $("[id*=hfSelected]").val(selectedText.join(','));
        });
    });
</script>

Namespaces

C#

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

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Code

C# 

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        string conStr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        SqlConnection con = new SqlConnection(conStr);
        DataSet ds = new DataSet();
        try
        {
            con.Open();
            string sql = "SELECT CustomerID, Name FROM Customers";
            SqlDataAdapter da = new SqlDataAdapter(sql, con);
            da.Fill(ds);
            ddlCountries.DataSource = ds;
            ddlCountries.DataTextField = "Name";
            ddlCountries.DataValueField = "CustomerID";
            ddlCountries.DataBind();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message.ToString());
        }
        finally
        {
            ds.Dispose();
            con.Close();
        }
    }
}

protected void Filter(object sender, EventArgs e)
{
    int inserted = 0;
    foreach (string item in hfSelected.Value.Split(','))
    {
        using (SqlConnection con = new SqlConnection())
        {
            con.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection = con;
                cmd.CommandText = "INSERT INTO Details (ITEMID,DETAIL) VALUES(@ITEMID,@DETAIL)";
                cmd.Parameters.AddWithValue("@ITEMID", TextBox1.Text.Trim());
                cmd.Parameters.AddWithValue("@DETAIL", item.Trim());
                con.Open();
                inserted += cmd.ExecuteNonQuery();
                con.Close();
            }
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim conStr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Dim con As SqlConnection = New SqlConnection(conStr)
        Dim ds As DataSet = New DataSet()
        Try
            con.Open()
            Dim sql As String = "SELECT CustomerID, Name FROM Customers"
            Dim da As SqlDataAdapter = New SqlDataAdapter(sql, con)
            da.Fill(ds)
            ddlCountries.DataSource = ds
            ddlCountries.DataTextField = "Name"
            ddlCountries.DataValueField = "CustomerID"
            ddlCountries.DataBind()
        Catch ex As Exception
            Console.WriteLine(ex.Message.ToString())
        Finally
            ds.Dispose()
            con.Close()
        End Try
    End If
End Sub

Protected Sub Filter(ByVal sender As Object, ByVal e As EventArgs)
    Dim inserted As Integer = 0
    For Each item As String In hfSelected.Value.Split(","c)
        Using con As SqlConnection = New SqlConnection()
            con.ConnectionString = ConfigurationManager.ConnectionStrings("constr").ConnectionString
            Using cmd As SqlCommand = New SqlCommand()
                cmd.CommandType = CommandType.Text
                cmd.Connection = con
                cmd.CommandText = "INSERT INTO Details (ITEMID,DETAIL) VALUES(@ITEMID,@DETAIL)"
                cmd.Parameters.AddWithValue("@ITEMID", TextBox1.Text.Trim())
                cmd.Parameters.AddWithValue("@DETAIL", item.Trim())
                con.Open()
                inserted += cmd.ExecuteNonQuery()
                con.Close()
            End Using
        End Using
    Next
End Sub