Hi arujdevia,

Refer below code.

 HTML

<form id="form1" runat="server">
    <script type="text/javascript" src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js"></script>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" />
    <script type="text/javascript" src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.16/js/bootstrap-multiselect.min.js"></script>
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.16/css/bootstrap-multiselect.min.css" />
    <script type="text/javascript">
        $(function () {
            $('[id*=lstFruits]').multiselect({
                includeSelectAllOption: true
            });
        });
    </script>
    <asp:ListBox ID="lstFruits" runat="server" SelectionMode="Multiple">
        <asp:ListItem Text="Mango" Value="1" />
        <asp:ListItem Text="Apple" Value="2" />
        <asp:ListItem Text="Banana" Value="3" />
        <asp:ListItem Text="Guava" Value="4" />
        <asp:ListItem Text="Orange" Value="5" />
    </asp:ListBox>
    <asp:Button Text="Submit" runat="server" OnClick="Submit" />
</form>

Code

C#

protected void Submit(object sender, EventArgs e)
{
    string message = "";
    foreach (ListItem item in lstFruits.Items)
    {
        if (item.Selected)
        {
            message += item.Text + " " + item.Value + "\\n";
        }
    }
    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('" + message + "');", true);
}

VB.Net

Protected Sub Submit(sender As Object, e As EventArgs)
    Dim message As String = ""
    For Each item As ListItem In lstFruits.Items
        If item.Selected Then
            message += item.Text + " " + item.Value + "\n"
        End If
    Next
    ClientScript.RegisterClientScriptBlock(Me.GetType(), "alert", "alert('" & message & "');", True)
End Sub

Screenshot