We have two sections on our tracking area to make it easy to add accurate Facebook tracking info.
Navigate to the settings tab (this only works on pro plan and above).
In the Tracking tags / pixels on ALL pages section, input your base Facebook pixel code.
In the Tracking Tag for Thank you Page input your event code for "lead" or whatever code you want to add in there.
It looks similar to the code below, but may be slightly different depending on your Facebook pixel.