Nothing to write in between. Use this way:
<iframe name="forum" src="http://mysite.com/index.php" frameborder="0" scrolling="yes" align="top" width="100%" height="100%" marginwidth="5" marginheight="5" ></iframe>
Syntax:
<iframe>...</iframe>
Attribute Specifications to Adjust Appearance and Behavior:
src="(URL of initial iframe content)"
name="(name of frame, required for targeting)"
longdesc="(link to long description)"
width=(frame width, % or pixels)
height=(frame height, % or pixels)
align=[ top | middle | bottom | left | right | center ] (frame alignment, pick two, use comma)
frameborder=[ 1 | 0 ] (frame border, default is 1)
marginwidth=(margin width, in pixels)
marginheight=(margin height, in pixels)
scrolling=[ yes | no | auto ] (ability to scroll)