First the lower kick panel should be removed. Inside there is a disconnected connector, connect it and the interior lights start blinking.
Sit in the car, close the doors, press and hold the central locking "lock" button, you will hear a confirmation beep.
Press the remote "lock" button on the fob twice. Doors locks will cycle between locked and unlocked. You can repeat with additional remotes.
Release the central locking “lock” button.
Disconnect the connection on the kick panel and test the remotes.
Both an used remote from an Impreza and a chinese remote from AliExpress worked.