Hello Daniel,
the Devices App API for License rental is not documented (was not intended to be public).
However you can reverse engineer the messages sent from Devices App to the Devices App Service and figure out the required messages.
Not sure if you want to manage only licenses for your own app, but the handling of the Licenses is always a read/write sequence - you read the licenses dataset, update the dataset with required changes and write it back.
Your App will require an access to the Devices App to be able to use the API.
Best Regards
Andreas Fink