That is what tips you off that this is Basic Auth. The -u part signifies Basic Auth, and then the colon at the end tells you this is being sent as a username. In normal Basic Auth, you send the username and password in the following format username : password so if you see a -u then you know that anything to left of the colon is the username and to the right is the password. In this example, the API key is sent as a username.