Instant reply is a reply automatically sent to the first message your page received from someone. It is disabled by default on your Facebook page.
If you wish, you can enable it, but you must be a moderator, admin or editor to do so.
Check out these steps to complete this task on a PC.
Visit your Facebook page.
On the left sidebar, click Inbox.
At the top, click on Automated Responses.
Enable Instant Reply in the left sidebar.
That’s all there is to it.
Note: You can disable the instant reply by disabling it.
Here’s how to customize your instant reply message:
Simply click Edit Message.
In the text field, you can edit the message to your liking.
You can now see the preview. Use the “Preview in Messenger” button at the top right to see how your message will look in the Messenger.
Once you’re satisfied with the message, just hit the Save button.
Note: The “Away” message will be sent if you have the instant reply turned on and the Facebook page status is set as away.