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: