Gereate a Facebook page and Go to Facebook page setting as shown image below-
Now in the Setting page, you should see at the left section tab menu of the page, then find the link of “messenger platform” tab and click on that tab link as shown image below.
Now you should see the Customer chat plugin section in the page, click on that section as shown in the image above.
After clicking there it will moved the page to setup of the customer chat plugin. Now you should click on set up button as shown image below: