Actually, Steve pretty much explained it all.
Go to your Admin CP (control panel).
Under
Portal || Custom Islands click on
Add Additional Custom IslandClick on '
EDIT' and enter your text/code where it says...
Body of the custom box hereNow click on
Forums/ Forum Settings then
[Details]Now click on the
Custom Inserts tab.
Finally select you new island from the
Custom Island Insert...and there ya go