Please note: the reminder can only be sent 5 minutes before the webinar. 

To enable FB reminder, first of all, go and edit your webinar

Click on Scheduling in the menu

Then in Easy - Scheduling, scroll down, in Webinar Reminders section, find Stealth Facebook Reminders, and check that option. You should now see something like this

Here is a quick video on how to Link Facebook Pages

After you are done adding your page, go to Customize menu. 

Scroll down until you find 5-Minute Facebook Reminder

Click on configure and edit the message to what you want. Please make sure not to change the webinar link custom field. Once you are done, hit Save