How can I append item to multi select list with script?

Let`s say I do have a multi select list containing initially "mike", "tom" (from editing field->enter choices). I would like to append "denise" to the same multi select list by using a script. Below code does not append, it erases mike & tom. Any suggestions? Thanks!

        on add
        {
            on load
            {
                multi:ui.add("denise");
            }