Yes, I did try that. In my case I see my United FF number there with no way to edit it.

I think in the example in post 4 it's either a brand new reservation, or it's to add a FF number if none exists.

In my case, as explained above, reservation was made months ago via corporate TA who added, rightfully then, my united number.

I guess I have to change it at the airport. I'll try calling once.