STEX API v3 uses OAuth 2 to authenticate users for private endpoints. For most cases it is enough to generate S2S token and use it in your requests.
Please refer to this article to learn how to generate the S2S token.
When you have S2S token you now can use it in postman. It should be used as a Bearer Token:
That’s it. Now you’re ready to send API requests through the Postman to the endpoints placed in requested scope(s).
Please be careful with scopes. Consider using different access tokens for different scopes, especially for withdrawals.