Azure AD getting authorization code

Hi
I'm have developed a new rest API using Azure Function.
I want it to be secure.
I have set up the App registration using this document:
https://docs.microsoft.com/en-us/azure/app-service/configure-authentication-provider-aad

And when I try to authenticate a rest request using this document
https://docs.microsoft.com/en-us/azure/active-directory/develop/v1-protocols-oauth-code

Instead of getting authorization code I get the “Sign in to your account” web page.

The request I made contains the following keys:
client_id, response_type, redirect_uri, response_mode, resource, state.

I used Postman to send the post request.

What am I doing wrong?
Is there a wiki or page that can demonstrate the whole process and best practice.

Thank anyone that can help
Izia