General developer forum

I would like to add two side-by-side multiselect elements to my mform where one is populated with items and the user selects the item clicks remove to move the item to the other mutliselect. I can do this in html and js but I don't see an element in the Forms API. How is this done? I see it in Moodle core on the define roles or webservices pages. But the code doesn't read easily enough to know how it is being done. Any helpful suggestions?