Adding an authenticator app without the ability to scan QR codes (i.e. the device doesn't have a camera) is impossible.
Despite the fact that there's a key displayed (alongside the QR code) when adding an authenticator to manually enter it to the app, the required detailed settings (Hash Algorithm, Number of digits, ...) are not shown.

This includes both the Authenticator page in Account management and also Login screen (i.e. when required action is triggered).