Add callback function for curved UI button

I was following along with this tutorial for adding a curved UI in a Quest 3 application: https://developers.meta.com/horizon/documentation/unity/unity-isdk-create-ui but I'm unsure where and how to add a callback that runs when one of the individual buttons in the UI canvas is pressed. It's not as obvious in this case as it is with say the Keypad from the PokeExamples sample scene. I have another GameObject with a function I want to run as a callback, with the behavior changing depending on which of the individual buttons is pressed