Im not sure if its going to be possible for you to add it to a Generic Content.
You may have to make a user control that contains the <iframe> and place that on the page.
Create a new file using your favorite editor, i used notepad.
Add this to the file:
|<iframe src="default.aspx?username=<%# HTTPContext.Current.User.Identity.Name %>"></iframe> |
dont forget to change the target file.
Save this with any file name you like, just make sure .ascx is the file extension.
You can then login to siteifnity and upload this control for use.
When editing a page, choose "Add Controls" , at the bottom there will be an "Upload Control" link.
Just fill in the simple questions. If it asks you to uplaod a codebehind file, click cancel.
You should then be able to place this on your page and get your desired results.