Configurable to have the chat box open on load or not (showing the Facebook Messenger Icon only).
The only thing you need is your Facebook Page ID which can be found with following instructions:
To find your Page ID:
From News Feed, click Pages in the left side menu.
Click your Page name to go to your Page.
Click About in the left column. If you don't see About in the left column, click See More
Scroll down to find your Page ID below More Info.