How to assign a user to a specific "index"?

Question

Access rights

There is an API, where one of the input parameters is 'index', authorized users have access to the API. How to correctly assign a user to an index so that you can make a request to the index that is registered for the tobl?
Example:
The user has indexes "2" and "3", if he makes a request to the API and writes 2 or 3 in the 'index' parameter, then the API returns a response, if a different number, then returns a failure