My flight reservation for business was made through a travel agency's website where I couldn't put my number in. I don't want to contact them with my question.

Is there a way to add my personal frequent flier number to the existing flight reservation, or should I wait till the flight is over, and contact United?

(I added our FF numbers to Hawaiian when our older son made reservations for everybody, but on United all I can find is the seat change option)

You can call them. Or you can have them enter your number when you check in. Better not to wait til after the flight.

Sorry, never mind I found it!

After keying in the confirmation # and last name, it opens your flight details. Scroll down to "edit traveler info" and on that page at the bottom there is space for FF number.

Thank you, Belinda!