Exact way of entering API key

Hi everybody.

Hope some of you can help. I was reading a lot of existing examples on how to get data from API using API keys but most of the articles here are unclear on this topic as I am new to APIs.

I need to get data from this API 
https://apidocs.chargebee.com/docs/api/subscriptions?prod_cat_ver=2#list_subscriptions

and the Authentification method should be simple - via API key and I have one.
https://apidocs.chargebee.com/docs/api/auth?prod_cat_ver=2

When I go to GET DATA > WEB > advanced
and enter all the necessary, this part is challenging me - I get  "A web API Key can only be specified when a web API key name is provided" - so I am not sure what is the exact and correct way to enter it and where. All other answers on this topic are pretty fuzzy and without some easy-to-follow solution.

Can you please help? 

Thank you very much.