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):
In the pop-up window select Web and then click on Get started:
Then select Facebook Pixel and click on Connect:
Name the pixel and click on Continue:
You may close the following pop-up.
Copy the pixel ID number in the upper left: