Short answer: It is not possible through Apple Business Manager and MDM to buy in-app purchases. I went through the process of trying to work this out. I ended up on the phone with support, who confirmed it was the case. The two options I was given was to either use a standard customer account for the purchases (we only needed for two iPads). Otherwise to go B2B and purchase a version of the app already unlocked and upload that as Custom App via MDM