On this trip from Croatia on a UA award, Croatia segments before going LH FRA-BOS, I remember seeing a seat map including extra-legroom seats for a price for the LH site, but wanted to see that we would actually get to Croatia before getting seat assignments on the TATL flight. Now we're there, and the UA and LH sites seem to say no seat selection until checkin the day before. Is there a trick to being able to select the seats online, or can or should it be done by phone?