To assign app manager rights to a user as administrator, follow these steps:
Open the app Administration.
Open the menu item User Management > Users.
Open a user from the list with a double-click.
Switch to the tab App Manager.
In the left column of the table, select the service(s) for which you want to define the user as an app manager.
Assign the service(s) to the user using the arrow key.
Find more information here: