choose player startはC++上の関数だな、BPからはアクセス無理そう

↓のスレッドが参考になるんじゃない?
https://forums.unrealengine.com/showthread.php?2107-Come-Learn-Blueprint-Multiplayer-with-me!-%28aka-Tom-s-a-Glutton-for-Punishment%29&;p=12895&viewfull=1#post12895