Step 1:

Set up a ‘Google Tag Manager’ account: https://tagmanager.google.com.

Step 2:

Click on the ‘Tags’ tab and click ‘New’

Step 3:

Select ‘Custom HTML’ tag and enter a name for your tag.

Step 4:

Create a Facebook business manager account. https://business.facebook.com/

Step 5:

Go to the Facebook Events Manager and select ‘Add Event’: https://business.facebook.com/events_manager/

Step 6:

Select ‘Install Code Manually’ and copy the code snippet.

Step 7:

Past the code snippet into the HTML container you have created in Tag Manager.

Step 8:

Select the trigger to be ‘All Pages’ and click save.

Your tag should now show up in the list.