We don’t have a sample/quickstarts for xamarin quite yet.
We had a similar question here:
Which illustrates using the OpenID connect / OAuth 2.0 pieces from xamarin. Does that help?
We have documentation on the calls to the token endpoint here: Implement authorization by grant type | Okta Developer
Also, our postman collection does show how to validate this with Postman (with interceptor):
https://app.getpostman.com/run-collection/4adca9a35eab5716d9f6