YEAH ! I have found a workaround. While in the "html" tab, I inserted manually the code
<IFRAME id=login style="WIDTH: 210px; HEIGHT: 99px" align=center marginWidth=0 marginHeight=0 src="form.html" frameBorder=0 scrolling=no onload=IframeOnLoad()> </IFRAME>
This will call the file "form.html" and make it appear (and work).