<%@ Register TagPrefix="telerik" Namespace="Telerik.Web.UI" Assembly="Telerik.Web.UI" %>
 
<telerik:RadWindowManager runat="server"></telerik:RadWindowManager>
 
<asp:TextBox runat="server" ID="TextBox1" ReadOnly="true" />
<asp:TextBox runat="server" ID="txtTestingEmail" Style="display: none;" />
 
<telerik:RadButton runat="server" Text="RadPrompt" ID="RadButton1"
    OnCommand="RadButton1_Command" CommandName="RadPrompt" />
 
<asp:Button Text="Plain Button" runat="server" ID="btnTestFirst" OnClick="PlainButton_Click" />
 
<script>
    function promptCallBackFn(arg) {
        document.getElementById('txtTestingEmail').value = arg;
        document.getElementById("btnTestFirst").click();
    }
</script>

protected void RadButton1_Command(object sender, CommandEventArgs e)
{
    RadPrompt("Please input the testing - E-Mail?", "promptCallBackFn", 350, 230, null, "Test-Newsletter", "");
} 
protected void PlainButton_Click(object sender, EventArgs e)
{
    TextBox1.Text = txtTestingEmail.Text;
}