This article will show you how to enable and add your NFC payload in the PassKit Portal.
This article deals with adding the payload to the pass only, for setting up and using your pass with an Apple VAS or Google Tap and Go terminal please see the instructions here.
Open your project and navigate to the designs section
Click the edit icon for the design you wish to add the payload to
Click on the barcode tab
Scroll down to the bottom and check 'Use NFC'
In the Payload box, add the payload you wish to use
Make sure to click save when finished