External API Rest API call not working

When we make external api service call to generate token or post data getting origin null error. I understand there is article how to make 3rd party api call but it not that clear to implement.

Also, this error is not only for generating token but if we pass token directly still getting same error for all api call.

Any idea how can we fix it?

We using 3rd party API and don’t have control change CORS.

Regards Yagnik