In order to make API calls against your project you must provide your authentication token in the header parameter Ocp-Apim-Subscription-Key of each API call.1
Obtain your authentication key by logging into your Fitabase project, click on "API" from the project dashboard and then "Launch API Portal"
Select "Profile" from the right corner menu.
Select "Profile". You'll then be able to see 2 keys (Primary and Secondary). Each are active and can be regenerated at any time if you have reason to believe their integrity has been compromised.
Keep these keys in a secure place! If you believe they have been compromised, immediate log back into your Fitabase project and regenerate them which will invalidate the previous keys.