HOW TO: insert record with primary key from login

Hi, here is my situation (this has to be very common)... BUT i can't figure it out:

1. a "user" logs in via built in security
   
2. user goes to the order table (just any table) and adds a new record
   so, CUSTOMER table has cust_ID and this is also the security table.
   ORDER table has order_ID (auto gen primary key)
   WHERE cust_ID is inserted into the ORDER table....
   HOW and WHERE do I get the cust_ID from the login event and get it inserted into ORDER table when a new order record is created?
   this is easy, but I dont' get where, how, what events, what keys/values/arrays, etc to use...
   help <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=21987&image=1&table=forumtopics' class='bbc_emoticon' alt=':)' />
   THANKS