All widgets have a main.xml configuration file that you can look at, and also edit if you need to change something in the code.
Kubuntu / Plasmoid widgets are located in the following directory
/usr/share/plasma/plasmoids/
I recently added the Web Browser widget to the bottom panel in Kubuntu. I wanted to change the start up page that appeared when the widget was opened, but there was nothing in the settings to change do this, so I wanted to see if there was a something I could change in the code.