There's nothing in the Fusion API to support this. It's all done by the add-in, usually using some type of web API to query and determine what type of license is being used and then behaving appropriately for that type of license. If you deliver your app using the Autodesk App Store, you can use it to determine if the current user has purchased the add-in or not. The App Store has an web API that supports that, but then once you have that information it's still up to you to use that information to have your add-in behave appropriately.
Another option is to do the licensing yourself. There are commercial licensing applications you can use or you can write your own.
But, to reiterate, this is all independent of Fusion and you won't find anything in the API to help except the ability to get information about the current user to know who is using your add-in.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com