It is possible to embed a user control in a datatype form. To embed a user control:

1. Create a user control. (You can use the following example, in which we create a control that renders iframe.)
   
   Sample.ascx:
   
   

   <%@  Control Language="C#" Inherits="Composite.StandardPlugins.Forms.WebChannel.UiControlFactories.ContainerTemplateUserControlBase" %>
   <%@ Import Namespace="Composite.ConsoleC1.Forms.WebChannel" %>
   <%@ Import Namespace="Composite.Plugins.Forms.WebChannel.UiControlFactories" %>
   <script runat="server">
   private  void    Page_Init(object  sender, System.EventArgs e)
   {
   
   }
   </script>
   <iframe src="http://composite.net"></iframe>

2. Add this control at the following path:
   
   ~/Composite/controls/FormsControls/FormUiControlTemplates/Containers/Sample.ascx
   Please note that the path might be different in your case and you'll have to adjust it to this control's path in Composite.config.
   
   IMPORTANT NOTE: Always create a backup of configuration files before any changes!
3. Locate the <Composite.Forms.Plugins.UiControlFactoryConfiguration> part in the Composite.config file:

   <Composite.C1Console.Forms.Plugins.UiControlFactoryConfiguration>
     <Channels>
       <Channel debugControlNamespace="http://www.composite.net/ns/management/bindingforms/internal.ui.controls.lib/1.0" debugControlName="Debug" name="AspNet.Management">
         <Namespaces>
           <Namespace name="http://www.composite.net/ns/management/bindingforms/std.ui.controls.lib/1.0">
             <Factories>
               <!-- add your control entry here-->
             </Factories>
           </Namespace>
         </Namespaces>
       </Channel>
     </Channels>
   </Composite.C1Console.Forms.Plugins.UiControlFactoryConfiguration>

4. And add your control under the <Factories> element as in the example below:

   <add userControlVirtualPath="~/Composite/controls/FormsControls/FormUiControlTemplates/Containers/Sample.ascx" 
        cacheCompiledUserControlType="true" 
        type="Composite.StandardPlugins.Forms.WebChannel.UiControlFactories.TemplatedContainerUiControlFactory, Composite" 
        name="Sample" IsTabbedContainer="false" IsFullWidthControl="true" />

5. Now open your datatype using the Edit Form Markup command in the context menu and add your control to the desired location within the <FieldGroup> element: <Sample></Sample>
   
   You may use the following markup as an example. (This datatype consists of one field of the string type and unlimited length.)

   <cms:formdefinition xmlns:cms="http://www.composite.net/ns/management/bindingforms/1.0" xmlns="http://www.composite.net/ns/management/bindingforms/std.ui.controls.lib/1.0" xmlns:f="http://www.composite.net/ns/management/bindingforms/std.function.lib/1.0">
     <cms:bindings>
       <cms:binding name="Id" type="System.Guid" optional="true" />
       <cms:binding name="NewField" type="System.String" optional="true" />
     </cms:bindings>
     <cms:layout>
       <cms:layout.label>
         <cms:read source="NewField" />
       </cms:layout.label>
       <FieldGroup>
         <InlineXhtmlEditor Label="NewField" Help="" ClassConfigurationName="common">
           <InlineXhtmlEditor.Xhtml>
             <cms:bind source="NewField" />
           </InlineXhtmlEditor.Xhtml>
         </InlineXhtmlEditor>
         <Sample></Sample>
       </FieldGroup>
     </cms:layout>
   </cms:formdefinition>

   (In case you need to adjust the size of the iframe used in our example, please use the <PlaceHolder> element instead of <FieldGroup> and set the height and width attributes of the iframe located in the Sample.ascx file.)