How do I generate Facebook Page Access Tokens for subscribing a page to a webhook

Question

I want to be able to do this programmatically. I am currently using the Koala gem like this

page_token = @user_graph.get_page_access_token(page_id)

but page_token doesn't work when I use it to subscribe the page to my messenger bot. However, the page access token generated from the token generation section of messenger works.