By default when you type in 192.168.88.1 or http://R1 into your browser's interface you will be able to access it immediately.
However, if you are in a busy household or have an unsecured WiFi network (no WiFi password) you may not want people being able to change your settings.
You can enable Interface Password Protection.
Once enabled, when you try to go to the interface you will be presented with this login box:
Login with the username and password you set to access the interface.
If at any time you forget the login details you can do a factory reset using the button at the back of the router. See more information on this here.