facebook graph api : subscribe someone else's page to my app - node.js

Question

I am creating a Facebook bot using which user can chat with the Facebook page.
Following is the scenario:
Allow user to login with his Facebook account along with required
permissions
Show user list of his pages
When user select on of the page; subscribe MY FACEBOOK APP to
that page
Setup webhook for app
Send-Receive message to/from user's Facebook page
I am having trouble with 3rd point. I tried to subscribe app using following code as mentioned in facebook docs
FB.api(
'ACCESS_TOKEN',
'/<PAGE_ID>/subscribed_apps',
'POST',
{"subscribed_fields":"[\n \"feed\"\n]"},
function(response) {
// Insert your code here
}
);
and it returns following response:
{
"success": true
}
The problem is- I don't know which app is subscribed for this page; I can have 100 apps. Graph api don't have provision to provide app id which need to subscribe against user page.
This is facebook doc for subscription

Answer 1

In order to get page access token for app, you need to use same app to login user and to subscriber to user's page.
This makes total sense as user is granting our app to their pages and provide us access token with the reference of our app.
Solution :
Allow user to login with his Facebook account along with required
permissions and your Facebook app
Show user list of his pages
When user select on of the page; subscribe YOUR FACEBOOK APP to that
page using page access token obtained in step 2
Setup webhook for app
Send-Receive message to/from user's Facebook page