Businesses whose Messenger experience is provided by a service provider, will not have access to the page token generator in app settings because they do not own the Facebook app. In this case, you may add or remove whitelisted domains via Page settings. To whitelist a domain, do the following:
Click Settings at the top of your Page
Click Messenger Platform on the left
Edit whitelisted domains for your page in the Whitelisted Domains section
How to file Facebook page ID
Open your Facebook page.
Click the About tab.
Scroll down to the bottom of the Page Info section.
Next to Facebook Page ID, you can find your page ID.