Once the bot is enabled you need to start a chat with `Hangouts bridge bot` with handle `@hangoutsbot:YOUR_DOMAIN` (where `YOUR_DOMAIN` is your base domain, not the `matrix.` domain).
Send `login` to the bridge bot to receive a link to the portal from which you can enable the bridging. Open the link sent by the bot and follow the instructions.
Automatic login may not work. If it does not, reload the page and select the "Manual login" checkbox before starting. Manual login involves logging into your Google account normally and then manually getting the OAuth token from browser cookies with developer tools.
You can learn more about authentication from the bridge's [official documentation on Authentication](https://github.com/tulir/mautrix-hangouts/wiki/Authentication).