You can add a Tab to a Form by opening the Main Form Definition window. Only users defined as System Admin, or a Full user with the specific module "admin" role, can access Main Form Definition.
In your selected sub-module you can click the Cogwheel icon to open the Main Form Definition window.
Adding a Tab is as simple as clicking the “+” icon in the Main Form Definition window.
After giving the Tab a name and clicking OK will add the Tab to the form.