User Authentication for webhook to MailChimp

I'm trying to set up a webhook to post email subscriber data to a MailChimp account using the MailChimp API. You need to be able to send a user authentication request but I can't figure out how to do this in QuickBase. I've tried adding a header with the key "Authorization" and value of a username and API key, but it doesn't work. 

The error I get back is always: 

{"type":"http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/","title"... Key Missing","status":401,"detail":"Your request did not include an API key.","instance":"cf324650-a4c5-420e-80b1-e62703b743fc"}