I don’t know about the current state of the MG switch branch, but @Tom and his company are working on it (I think they’re porting both Towerfall and Stardew Valley, but not 100% sure). I suspect getting access to MG Switch will work the same as for PS and Xbox, i.e. you get a developer license, then contact Tom for access to the MG repo (no further requirements that I’m aware of).
If you’re making a new game, I think it’d be easiest to target desktop at first. That way you have something to show when you apply for a Switch dev license and porting to switch should be relatively easy once the MG Switch back end is a bit further in development.