AirPort base station

When You first set it up if You selected remember in Keychain you should be able to get it from there.

Go to Applications~Utilities~Keychain~Open Keychain and select login passwords-enter Airport in search. Click on Airport Base Station to open select show password; you will be prompted to enter device password and select allow and the password will be shown.

Hope this Helps.

If you don't remember the Base Station Password, you can perform a Soft Reset to reset the password.

How to reset your AirPort base station - Apple Support