The easiest way is using the ‘Add New Place’ feature inside Studio.
This is also not possible:
Roblox made it impossible to set your starting place to something else. (It was possible previously.)
If you currently want to change your starting place, you have to back the current one up, overwrite the starterplace, and upload the backed up one to a new place (if you want to keep it).
That’s provided the supposed places you intend to add to the game are not Start Places. Such places that can be added through the Configure Game page are remnants of removed Start Places before the update occurred. No new developer will have a use for this tab. Only old developers who removed Start Places before would have any real reason to dig on this page to add loose places.
I actually was trying to do this, for my new game so you load in the lobby and then when you teleport to the game istself it wont show up in recently played. I am hoping this is how you do it!