You need to do the following steps:

Log in to your Facebook Business account

https://www.facebook.com/events_manager

In the account menu (top left), select the Events Manager tab and then click on Connect a Data Source (when creating a new pixel):

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/b3192974-ca7f-4161-a9e5-152c4bb079e8/pixel_id1.png

In the pop-up window select Web and then click on Get started:

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/f30d3ff7-b9e5-4729-9184-4336be2687b9/pixel_id2.png

Then select Facebook Pixel and click on Connect:

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/1b3c5e70-47da-42ca-9a25-3faaad5a9d4e/pixel_id3.png

Name the pixel and click on Continue:

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/9243cc05-6dfe-4619-9109-8ffc41825f6f/Pixel_id4.png

You may close the following pop-up.

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/c0ba2b60-6e9e-4717-bf01-2fdc061d5e96/Pixel_id5.png

Copy the pixel ID number in the upper left: