How can I use Facebook Messenger Platform's webhook to subscribe to the Page messages of other users?

I am creating an application using which users will manage their Facebook messenger and post comments on a page. I have implemented the Facebook Login which asks the user for page_messaging permission. I have also configured the webhook and it is working fine.

Now, Facebook documentation states the following

Generate a Page access token to start using the platform APIs. You will be able to generate an access token for a Page if:

1. You are one of the Page admins, and
2. The app has been granted the Page's permission to manage and access Page conversations in Messenger.

So, how can I make my webhook listen to the messages of the page that have granted me permission to manage and access the page? I have looked for hours trying to find this in the documentation. No Luck!