Well I've known how to do this for about 5 weeks now and it's not to hard.
You can choose the map you want to play on any game mode and be host of it too.
However due to the new update 1.14 being host now gives absolutly minimal advantage and some times other players with a weaker connention can slow down the badwith on you'r router causing the HOST lagging but nobody else.
To force host it is prefered that you have a good connection. around 75 - 100 ms ping.
The way to foce host is to start any game mode and look at the top where it says searching for game.
wait until it says ============== no games unavailable ================ and then you will be host,
IF IT GOES ONTO NEXT PING EXAMPLE FROM 50 TO 75 you won't be host, if at 50 ping it says 0/50 games found and then says no games available you will be host. but if it goes to the next ping you will probably join another lobby.