Traveling from IAD-IST with a change of planes in MUC. UA business IAD-MUC and LH coach MUC-IST. I am 1k and have a Red Carpet membership. What's the appropriate lounge for me to go to during the layover?

Also, I assume since this is a code share that UA will issue my ticket on LH, right? Do they give me a seat assignment, or do I have to get that at the gate?

Thanks.