Your code is for the definition txt file which cannot be truly executed. It is for the rendering as placeholder for the actual control when the control (field) is being inserted in form designer.
You should have the custom control txt file like this:
<div class=“Field” id="_ROW">
<!-- <%@ Register TagPrefix=“muc” TagName=“MyIframeControl” Src="~/MyIframeControl.ascx" %> -->
<div class=“FieldCaption”><label class=“FieldCaptionLabel” for="_IFRAME">IFRAME:</label></div>
<div class=“FieldValue”><muc:MyIframeControl ID="_MUC1" class=“FieldValueInput” runat=“server” /></div>
</div>
And then prepare a .net usercontrol that has the following in the .ascx file that contains the actual iframe tag:
<%@ Control Language=“C#” AutoEventWireup=“true” CodeFile=“MyIframeControl.ascx.cs” Inherits=“MyIframeControl” %>
<iframe class=“FieldValueInput” id="_IFRAME" runat=“server” src=“http://[your web URL]” style=“border:none;” scrolling=“no” frameborder=“0” height=“60” width= “600”></iframe>
You can even make use of the code behind of the user control (.ascx.cs) file to configure your iframe properties dynamically:
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class MyIframeControl : System.Web.UI.UserControl
{
protected void Page_Load(object sender, EventArgs e)
{
//— configure your iframe here if necessary
}
}