Hi, Ralph:
If I may expand on Patrick's answer....
You can add another data connection to your form that receives data, and you can select your Access database as the source. Perhaps it would be easiest if you created a View in your Access database that returns the max AssyNo and the max of the other field you want incremented. Here is a blog post with more information on the max function in Access.
Then, use rules for your Submit -- you can do that if you go to Tools / Submit Options. Select to Submit via rules. Your first rule would be to query using your data connection to the view that returns the max AssyNo. Your second rule could be to set the form field value that you want populated with the AssyNo from your secondary data connection + 1. Then your third rule could be to actually submit the form.
Hope this helps!