I have one old IP.Board 3 forum, where I used this code, but I don't see any result: https://www.walkertek.com/google-translate-sample-page/

In browser console is:

Failed to load resource: the server responded with a status of 403 ()
translate-pa.googleapis.com/v1/supportedLanguages:1

When I click on it I see this page:

https://translate-pa.googleapis.com/v1/supportedLanguages

With this result:

{
"error": {
"code": 403,
"message": "The request is missing a valid API key.",
"status": "PERMISSION_DENIED"
}
}

Where I can add API? What should I do?