A slot is only used up by a Universe/Game, which is made up of 1 or more Places (only one of which can you click "Play" on - the rest you have to teleport to). You can have as many Places as you like in a single Universe. CreatePlaceAsync creates a permanent Place.
You should be able to CreatePlaceAsync using the command bar, though I haven't tried it myself. You will need to first configure some options for the place you're using as a template to allow scripts to use it as a template.
Equivalently, you can just upload a new place. If it is listed in its own slot, you can simply transfer it to the desired universe using the Universe configuration menu.
To get to these menus:
- Go to your profile
- Click the gear on the game you wish to configure
- You are now in the Place configuration menu.
- If you want to go to the Universe configuration menu, click on the "Games" option (it's under "Developer Products". It will say "This place is part of the Game: _____". Click that link.
- Enable "Enable Studio Access to API Services:"
- Click on "Places"
- Click "Add Place"
- You can now add a place you've manually uploaded to the universe
- If you click "Created Places", you can see all the places you've made using CreatePlaceAsync