How can I choose starting positions in multiplayer sessions? (pockets/middle)

Hey, I’ve got a question: How can I choose starting positions in multiplayer sessions (in standard maps)? I want to play with my two brothers and one of them wants to be in the middle position, but it didn’t work, he was always on the side.We tried to join the server in a different order, but it did not work.

Can you help me? Is there a solution for this problem? (and why is there no button to determine starting positions?)

Thanks, Michael