The mandatory app can be found in the "App Management." To find a mandatory app, click on the rack wheel icon and then choose "Apps". Here, you can find a list of all apps.
A tick icon shows if the app is marked as a mandatory app for some or all users.
How to display an app as mandatory
To display an app as mandatory, click on the pencil icon next to the app and scroll down until you find the "Visibility on the Launchpad" tab. Here, you can find the category "Mandatory app" with it's three options:
Display this app for all users in "My Apps". This app can not be removed by the user.
Display this app under "My Apps" for users matching specific criteria.
Choose an option and click on "Update" to save the changes.