put firebase-secrets.json (firebase service account) in project root directory. Run the shortener and let it generate config.json. Change databaseURL to your rtdb url.
{
"databaseURL": "https://example.firebaseio.com",
"rateLimiter": {
"window": 300,
"amount": 30
},
"port": 8008,
"localization": {
"loc-name": "URL Shortener",
"loc-rate-limits": "Rate limits apply: %1 shortens in %2 minutes",
"loc-long-url-box-p": "Enter long url address",
"loc-long-surl-box-p": "Leave empty to get random link",
"loc-shorten-button": "Shorten!",
"loc-long-surl-out-box-p": "There will be a shortened url",
"loc-404-error": "404 Not Found",
"loc-404-h2": "Nobody have ever shortened this",
"loc-404-shorten-first": "Be first to shorten!"
}
}