Originally Posted by
PiperAtGatesofDawn
If you decide to book through UA. You can call up UA - give them the UA PNR and get the LH PNR. Then you can use the LH PNR to book your seat on the LH website.
That's incorrect. If the ticket is not of a type which allows free seat selection (and many LH fares do not), it is not possible to purchase an ASR if you buy a UA codeshare. The LH software won't allow it. This has been discussed for years and at great length on the UA forum.