Facebook Messenger Chat Plugin on your website & Whitelisting your domain

1. Whitelist your domain by clicking on Settings -> Advanced Messaging

Always include both the versions, with and without www: https://www.domain.com and http://domain.com

2. Open Messaging -> Add Messenger to your website -> Get Started

3. Click Setup and choose Standard

4. Set language, confirm the domains, copy the code and email it to [e-mail]

5. Done :)