The-Ensemble:
What is in the sub-page, more fields to enter data in, or something else? Either way, you could edit the page template and throw a link in there.
You want the stuff entered NOT to go into their profile, but just into a PM?
garyrogers:
Kewl use of the hack

Sorry, no clue why it's doing that. Please restrict dropdown posts to the dropdown hack's thread, and vice versa
WARNING: allowing users to post links blatently in their profile like that screams for spammers to abuse it. What I'd do is to change it a bit, like so:
HTML Code:
<if condition="$post['field14']">
<tr><td class="vbmenu_option"><a href= "http://www.google.com/calendar/embed?src=$post[field14]" target="_blank">My Calendar from Google</a></td></tr>
</if>
and change the field to be a place for their email address only. That way, you know it'll go to the google calendar page. However, this may very well still be abused by crafty coders, although you could probably do some regex parsing on the field itself somehow to prevent this. No, I have no idea how to do it, just that it may be possible