Here is how to allow chat on your website.
1. Go to your Facebook Page Settings
Navigate to your Facebook page settings. The link will be in the top right of your page.
2. Find the “Messager Platform” tab.
Scroll down till you find the messenger platform tab.
3. Add your domain to the whitelist.
Tell facebook that your domain is whitelisted to use the messenger plugin by adding your domain to the whitelist.