Please follow the instructions here to generate the access token to set up Conversions API 

Tutorial video:

Step 1: Go to Facebook Events Manager, make sure you log in and choose your business account 

Step 2: Choose the Pixel ID you want to set up conversion API  

Step 3: Go to Settings, scroll down and click Generate access token under Conversions API section 

Step 4: Copy the token appearing, make sure you save it somewhere else as Facebook won't store it. 

Step 5: Paste the token access into our app