Facebook messenger is one of many chat tools you can add to your Flex website.
When implemented the chat icon will appear on your website similiar to this:
How to get the code for installation
Go to your business facebook page and click on settings
Scroll down to the Add messenger to your website section:
Click on the set up chat plugin and go through the steps, on the final step you need to copy the code and send to us, or email us firstname.lastname@example.org
You can now customise the rest of the Messenger plugin (you can also customise it once it is installed)
More detailed instructions and customisation