First, you’ll need to retrieve your Facebook page ID. You can do this by navigating to your page, clicking on the About tab, and scrolling to the bottom of the page. There, you will see your page ID (It should be a number).
Follow these steps to find Facebook App ID: click
Next, you’ll need to navigate to the "PAGE SETTINGS", then to the “MESSENGER PLATFORM” tab and whitelist your domain. Type in both the HTTP:// and HTTPS:// variants of your domain to ensure it works for 100% of users. Click the SAVE button.