I have created a dynamic selector list that is pulling the usernames and user ids from the #_users table. I need to assign the user id that is selected as the userid of the submitted form. I can successfully change the IDs in the entire column so I know I've mapped it correctly but my WHERE fails. I can't seem to get it to map the correct row. The only way I can think this will work is to set the WHERE to query the formID but when I use the place holder {last_insert_id}, the id does not get changed. I am wondering if I need to do use the $mappings variable in the "Scripts called after form has been processed field". If this is the case I'm not sure how to correctly write this type of syntax.
I will gladly post the final solution for others to use, I'd bet I'm not the only one who has run into this issue.
Please help if you can, any advice will be greatly appreciated.

This is an update.
I have decided that the update has to occur post form data so I removed the mapping and have placed the following code in my post form data area:

$db =& JFactory::getDBO();
$db->setQuery(UPDATE xside__rsform_submissions SET UserId = '.$_POST.' ORDER BY SubmissionId DESC LIMIT 1);
$db->query();

I ran a test query in phpmyadmin and replaced the ="" with an actual value and it successfully updated the last row so that worked there. But the above does not update the row when placed in the post form data area.
Does anyone see any reason this wouldn't work?

At a very quick drive-by glance, you are using the whole $_POST array in that statement as if it was a single value. That is/was almost definitely the problem.