Please look at the stage example scene and find the script named "STMEventInterpreter"! It has a custom event (<e2=myEvent>) that's used to spawn a "link" prefab overtop of letters. I believe this is better than any built-in system because it can be customized with any type of overlap method and do anything when clicked. The default prefab spawned uses OnMouseDown() and prints a debug message, and uses the script "STMSampleLink" to do this.
I keep getting this question, so I'll try to make this its own sample scene with a dedicated custom script for the next update! But for the time being I hope this helps.