The API approach has been available for a long time, but now the autonumber is available through the config. Jonas Rapp’s autonumber solution in XrmToolbox has been absolutely one of the best tools for this purpose so far, and not we might not need it any more. I belive the first release is missing some of advanced options that Jonas has, but it might work for most of us.
This is the release information regarding the PowerPlatform autonumber: Autonumber from MS PowerApps blog
NB! you can only change the field to autonumber via powerapps.com. The Dynamics classic doesn’t support this. Below is the name field from a new entity. For the first time in EVER I could change the field type!!:)