Well I finally found the answer.
It looks like you have to define the handler your self. So in your code add to the part where you define the listbox:
$listBox1.Add_SelectedIndexChanged({&$listBox1_SelectedIndexChanged})
Then where you want to call the handler you need to create the function. So add the code:
$listBox1_SelectedIndexChanged=
{
$textBox.text = "clicked on an item in the listbox"
}
If you are interested in the final project, let me know. It will be a few more days to finalize but I'm more then happy to share it.