Is there a way i can put the game in Full screen windowed mode before i launch it?

Your best bet would be to wait until after you've launched it and adjusted the settings before you add it as the source for your xsplit, that way you can adjust whatever video settings need such prior to actually doing so, but if you're worried and in doubt, it might be best to hop your stream for a moment them in order to set it up. Your viewers should understand that you'll be coming right back, and if you want, you can explain that you want to do everything you can to avoid a possible ban like what happened in the beta.

So lets say i was just doing a window capture of the desktop. It would not be capturing destiny 2 so that means i can adjust it in game then add the source? I do have a BRB scene that i could use whilst setting up. That'll work right?