This tutorial walks through the process of creating a custom Facebook event that fires when users watch an embedded YouTube video on your website.
The full tutorial (with tag code) is here:
https://www.jonloomer.com/custom-facebook-event-watched-youtube-video-gtm
The steps:
1. Configure the Video Variables in Google Tag Manager
2. Create a Video Trigger
3. Create a Tag
4. Test it with the Facebook Pixel Helper
I used the following article as inspiration and made some alterations.
Feel free to make your own alterations to fit your needs.