The API use HTTP Basic Auth.

HTTP Basic Auth

HTTP Basic access authentication is one of the easiest authentication methods and it's only safe with a secure SSL/HTTPS connection. The header generated is:

Basic {TOKEN}
where the {TOKEN} is the base 64 of the account_loing and api_key separated by a colon. In pseudo-code, it would be:
base64(account_login + ':' + api_key)

GET /inventory/product HTTP/1.1
Authorization: Basic aHR0cHdhdGNoOmY=