When a customer leaves you a message on your store, you can reply it via Facebook Messenger.
Make sure you have signed up an account with Modern app.
Make sure you have a Facebook Page.
1. Log in to your Modern account > "Widgets" > Add Modern to your website
2. Copy the code
3. Go to Admin > Apps > Featured apps > Search for Modern
4. Paste the code > Save settings
5. Back to Modern > Widgets > Create widget
6. Choose widget type
7. Change the setting & message > Create widget