Very easy on UAL out of Frankfurt. Go to the ticket counter, the additional services section, and ask to list for the jumpseat. They are familiar with it. Get listed and PAY THE TAXES (!!!). If they do not mention taxes then ask them about it. Trust me you do not want to get to the gate and then have them remember the taxes. They cannot do taxes at the gate so you have to go back to the ticket counter. In FRA that can be quite a hike. Taxes were 81.5€ last time I did it.