I was wondering how on earth I can join a specific slot in an open lobby. Like in a custom scenario where there is slot A B C D etc how do i say I want to join C specifically - when i click join game it just gives me one randomly.

Join and decline a game until you get the slot that you want.

You can ask the host to assign the slot you want.

If it's a FFA, you'll have to quit and rejoin until you get it or request the host to put you on said slot. Both ways, it requires some luck.