Here you can:
and from there choose one of your branch lines and click on “Enable for Mendix Studio” (here ‘Main line’ is already set for Mendix Studio)
In Studio Pro, branch line manager can be found in menu Version Control | Manage Branch Lines.
You can find documentation here: https://docs.mendix.com/refguide/branch-line-manager-dialog#1-introduction and here: https://docs.mendix.com/refguide/collaborative-development#managing-branches
Hope that helps,