Kubernetes Service account authentication in Postman
Cant directly answer your question, but from what comes to my mind is that: Istio supports Token-based end-user authentication with JSON Web Tokens.
You should understand I havent tried to do that, but this looks for me very promising: Istio End-User Authentication for Kubernetes using JSON Web Tokens (JWT) and Auth0
And to test JWT-based authentication and authorization workflow thay exactly use Postman, as you prefer.