The problem with Sudlev's approach is that it requires property demotion to get the ID back into the form. And, that's not on by default nor is it a best practice.
I would rather suggest that you just do a double submit from the form using a rule that has an action to submit, another action to query and set the value, and a final action to submit again. I've responded to others with this technique in more depth. Please search for "double submit SharePoint ID" or something and you should find them. If not, let me know.
Thanks!