Button event is not working

Hello,
I hope someone could give me a hint how to solve the issue.
After hitting a button there should be a popup with a possibility to enter a invoice number.

This inoice number has to be updated on the selected entries on the list page.
My coding:
On the the function: "OnBefore":
params["invoicenumber"] = prompt('Pls enter the invoice number','');
On the Server: "OnServer":
for each val in keys

strUpdate = "update orders set invoicenumber = '".$params["invoicenumber"]."' where order_id=" &val("order_id")

CustomQuery(strUpdate)

next
But after hitting the button, nothing happen.
Does someone has an idea/hint for me?

Thanks in advance.

Chris