in console firebase, in the menu Authentification -> Templates
There is a sms verification template :
%LOGIN_CODE% is your verification code.
Meanwhile you cannot change the content of the message, you can change the app's URL mentioned in the SMS.
Go to Authentication > Sign-in Method > Authorized domains and add your own domain website.tld
Now edit the Firebase config object (in your source code) and update the authDomain key to the domain you linked earlier.
Send a new test message, you're good to go.