Access token with custom ids

Hello, I have this following code:

var client = new RestClient("");
            var request = new RestRequest(Method.POST);
            request.AddHeader("content-type", "application/json");
            request.AddParameter("application/json", "{\"client_id\":\"x\",\"client_secret\":\"y\",\"audience\":\"http://localhost:7071/api/CreateEvoucher\",\"grant_type\":\"client_credentials\"}", ParameterType.RequestBody);
            IRestResponse response = client.Execute(request);

And my question is, how I can generate access token with specific custom UserId. Because I don’t want store UserId in body but in access token and userId can be change in every new request