Xero OAuth 2.0 Call returning HTML as access token

Hi everyone,

I am attempting an Oauth 2.0 flow with Xero and Postman, as per the postman docs here

I’m following this documentation exactly as described but my call to generate a new access token returns a HTML page (A Xero Login page) instead of the token. This page is returned after I have requested a new access token, logged into Xero, authorised my application and been notified that my authorisation has been successful.

I’ve attempted this flow with different call back URIs and parameters with the same result. What am I doing wrong? Thanks.