B4J Question SOLVED: Basic Buttons on RPi Server

I have a very basic text HTML page which I use to monitor and debug my nonUI Raspberry Pi app.

I can pass parameters to it in the url and handle them.

I can also create a Button that appears on my page.

How do I persuade the button to do something?

I started looking through the Server Example which I can see does this. I can see the Handler in Guess, but where is the button defined?

I have searched the forum for 'Button' but struggled to find anything.

Thanks