Hi Ken,
There are 3 ways to do it;
a) As Robert points out, the docs show the "Generic" way to do it. This is the most flexible approach.
b) On the form itself, Advanced tab, you can set the action, and ID fields to default values. In your case set the default action to "Change" and the ID field to p_web.GSV('SessionValue')
c) duplicate the template stuff in hand-code (but there's no point doing it this way.)
Cheers
Bruce