Flying in October booked through third party (Viking) flying Lufthansa operated by United. I cannot figure out from the Luftansa website how to change seats on the flight from Frankfurt to SFO. If it is operated by United, how do I get the United confirmation number? I have the Lufthansa confirmation but I have not been successful in changing seats. thanks.

Viking should be able to give you the United confirmation number. Or, you can call United, give them the details, and they should give it to you. Then go to united dot com and use it to select your seat(s).

Have you entered your details on the United website to pull up the flights?

When I've booked through United, but flown a Lufthansa flight, UA has always provided both the UA confirmation numbers and the Lufthansa confirmation number on the United website.