1. Go to https://business.apple.com/ and log into your account
2. Once you're signed in, click Custom Apps in the left sidebar.
3. Next click on your app and then on the right, under "Buy Licenses", change Licence Type to Redemption Codes, then enter a Quantity and click "Get". Click confirm purchase, your internal app will be free to yourself.
4. Once the loading box disappears, on the same page, on the right, simply scroll down to "Redemption Codes" and click Download next to the most recent. This will give you an Excel file full of redemption codes which look like this: NLRTH77HWMKL
5. Now you should be able to follow the instructions here: https://support.cuttlefish.com/how-to-redeem-an-app-with-a-rede to be able to redeem the app.