This looks like a duplicate question of https://community.ringcentral.com/questions/108953/changing-user-licenses-from-api.html - please see the answer that this is not possible.

There is no API to create a license, you need to manually do that via the Service Web portal -> Billings tab. Here are the types of licenses we provide: https://support.ringcentral.com/article/RingCentral-License-Management.html and here is the tutorial on how to add one: https://support.ringcentral.com/article/Add-Licenses-in-your-RingCentral-Account.html

Btw, please do not duplicate cate posts with same questions, we can discuss it here or https://community.ringcentral.com/questions/108953/changing-user-licenses-from-api.html

Having said that, to provision a user (regardless of license as that is not an open in API), refer to Create User API : https://developers.ringcentral.com/api-reference/SCIM/createUser2