You authenticate each request by presenting your API key in the request header.
Authorization: Bearer {API_KEY}
For example, the beginning of a request using a fictitious key:
curl https://api-sandbox.synctera.com/v0/customers \
-H 'Authorization: Bearer 476d901b4-a264a79-9db9-96d3dfaafb732'
The examples in the Guides assume that you have set an environment variable apikey, e.g.
export apikey=476d901b4-a264a79-9db9-96d3dfaafb732
You also need to set baseurl, see the Environments section for more details.