To use some products of Facebook such as Customer Chat Plugin, you need to add your domain site into whitelist of your fan page. Through this article, I will show you how to do this. It just has a few steps.
Firstly, you should go to settings fan page.
Next, click on the Messenger platform menu on the left
Finally, scroll down to White-listed domains section. Enter your domain into the text box and save.
Note: your domain should activate SSL before added.