This article assumes you have already have an Apple Business Manager account. If not, you can create one by following this article: Setup Apple Business Manager Account

Login to Apple Business Manager and click your name, followed by "Preferences"

Then select "Enrollment Information"

On this page enable "Custom Apps" if its not already enabled. By switching the switch and clicking Save

Then copy (or screenshot) the Organisation Info - Name and ID - and send it to your Altitude Representative.

Once the app has been through Apple review it will appear on your Apple Business Manager account in the "Custom Apps" Section. Here you can manage the app the same way as Volume apps in the "Apps and Books" section.

Select the app and then the location, then the quantity you need to deploy, typically add 20% more just in case there are new devices added at a later stage and Apple and the MDM will manage these "licenses".