You need to scan the code using your banking app or a compatible payment app. Below are visual instructions for DBS Digibank and POSB Digibank:
a. if not logged in to the app, the link should be on the app homepage
b. if logged into the app, click on Pay & Transfer and the shortcut should be in the menu at the top of the page
PS: make sure your spending limit for PayNow is high enough by clicking on 'More' and then 'Manage Scan & Pay'