You need to add the variable to the end of your link.
You wrote this:
HTML Code:
<a href="search.php?$session[sessionurl]do=finduser&u=$post[userid]&starteronly=1">Threads Started</a>
Add it at the end like this:
HTML Code:
<a href="search.php?$session[sessionurl]do=finduser&u=$post[userid]&starteronly=1&forumchoice[]=xxx,yyy">Threads Started</a>
(Not tested - you may have to play with it.)