Basic access authentication is simply providing a username and password for a request via HTTP. This is the strategy that the Guardian API uses to authenticate itself when it calls external systems, such as notifying an external system when the employee has completed section 1 of the I-9.
If your integration includes receiving calls from Guardian, you will be expected to supply a username and password. Open the admin page in your Guardian installation to provide the username and password that the Guardian system should use for your systems.
To verify that your system is configured correctly, try this command: