Are you already using any Facebook SDK's on your website?
In case you are already using any Facebook plugins like - FB Share Plugins, FB Pixel, FB Like us buttons etc, please do make sure you whitelist your website Urls on your Facebook app. Please find the steps below to whitelist your website on your Facebook app.
Select your Facebook app that you are using on the website.
Go to Facebook login > Settings
Add our website url in 'Valid OAuth Redirect URLs'
What is a checkbox?
The checkbox plugin allows you to display a checkbox in forms on your website that allows users to opt-in to receive messages from your bot in Messenger. For example, you could include the plugin on an e-commerce website, where you wish to send receipts and order updates to the user.