Probably better would be /forum-view/12345/. Otherwise you need to write a parser to get to the 12345 part. Then you use an index.vuh file, grab the id like so:
set forum_id [lindex [ad_conn urlv] end]
while {[set index [ns_set ifind [ns_conn form] forum_id]] > -1} {
ns_set delete [ns_conn form] $index
}
ns_set put [ns_conn form] forum_id $forum_id
rp_internal_redirect forum-view
I've set this script up for testing. Try sticking ?forum_id=3456 on the end. It will remove this from the form and replace with the value 12345.