To modify the Function, right click on the button and select Properties.
Use the "window.open()" call to invoke a URL.
In the example below "window.open('http://ca.com/','_blank');" has been entered into the Function Field.
By using '_blank' the URL will be loaded into a new tab.
Now save this form and publish the changes. When the button is selected in Service Desk the user will be directed to the specified URL.