Once you order the app in Apple Business Manager (make sure that it's ordered to the correct location if you have multiples), the app will show up in "Jamf Pro > Devices > Mobile Device Apps" or "Jamf Pro > Computers > Mac Apps > App Store". Where it shows up depends on whether you're talking about iOS/iPadOS or macOS apps. If it doesn't automatically show up, you can go into either of those locations in Jamf Pro and click "New" at the top to add a new app. You can then search for the app that you added in ABM and add it to Jamf Pro. When you are deploying the app, make sure that you click on the "Managed Distribution" tab within the app in Jamf Pro and ensure that "Assign Content Purchased in Volume" is checked. If that box isn't checked, the user will be prompted to sign into the Apple App Store to download the app.
