Firstly you have to signup for VONAGE APIs 

A third-party service that connects you to your real-time, through Facebook. After signup, you can see a dash boar seen below. First, copy API Key and API Secret then click on the button shown in the image below 

After that you can see this page click on facebook messenger

After clicking on Facebook messenger you can see a new view as shown in the image below place this Url 
"https://gtchatpro.com/fbmessenger" in both fields of webhook.

Now click Send invite email then check your email it takes you to Facebook login after login you can see this page as shown in the image copy the key shown in the URL.

After that place API Key API Secret and this facebook messenger id in your admin dashboard profile section as shown in the image  

After clicking edit button you can see this 

Now you can see your messenger and ready too goo