I also book all my stuff on a corporate site.
I log into usair.com and pull up my itin. Click on the Send E-mail link on the left hand side fill out the form with my PNR and MP number and it gets added in 2 days or so. If you're booking a day in advance it might not work, but I usually find much less error this way. My MP number likes to repeat the same 2 numbers, then the last sequence uses different numbers and the agents in person would often mess up the rest of it.
I like to OLCI with my boarding pass with my MP# already on it to save me time at the airport.