Originally Posted by senorgreg

I had a UA award booking on Thai and Lufthansa. I was able to choose my seats on Thai and Lufthansa online.

First, log into your reservation on United.com. Near the top of your reservation page, use the link to obtain reservation codes for other airlines. I was only provided with the LH (Lufthansa) code. I then went to https://www.lufthansa.com/us/en/My-Bookings to choose my seats.

I was also able to use the same LH reservation code on the Thai site to also choose my seats, and following advice on this site, I could use the requests section to choose my meals, beyond the simple "vegetarian" etc.

Hope this helps others.