2262868 everyday users or Forum Support won't be able to generate credentials for you.
To get a Consumer Key and Consumer Secret for your app, you will need to submit an official application directly through the Garmin Connect Developer Program Portal.
A quick heads-up on security: Because PWAs run entirely client-side in the browser, please make sure you aren't hardcoding your Consumer Secret directly into your PWA's frontend code, as it can be easily extracted by users. You'll want to handle the OAuth handshake securely through your backend server.