This assumes you already have the Facebook pixel, if not, then follow these instructions.
Go to the Facebook Events Manager.
Click on the Settings tab for your Facebook pixel.
Scroll down till you find the Conversions API section.
Click Create Access Token.
Follow the guided creation by clicking Next till you get to Set Up.
Click Set Up.
Select your pixel and click Next.
Click Generate Access Token.
Copy the token and save it.
Click Next.
Click Done.