How to authenticate the Constant Contact to add a contact

Hi everyone,

I am following the documentation here: https://developer.constantcontact.com/api_guide/client_flow.html

I am trying to create an automation workflow in Python that allows me to add a contact using the Constant Contact API but I keep getting the authentication error hence the need to authenicate.

In the link provided above. I have done step 1 and received a 200 response but an HTML page as a response instead of a JSON.

The next step is step4, its not clear to me what code_verifier  and code  is. I dont know what values are required here.

More importnatly. step 2 is not clear to me at all. How do I get here?

Thanks for your response