I am flying LH from ORD to MUC. However, my first flight to ORD is on UA. I wonder how I can select seats when I check in on UA. I got checked in for the first segment, but I can not figure out what to do after that. Is there a number I can call? It is a UA ticket, and if it matters, the booking class is H. I would like to select seats as soon as possble to get a desirable seat but I can not figure out how to do this.