How to enable drop-down list to allow multiple selections

I have a form that has a repeat section, inside that is a row that has fields, one of the fields is a drop-down list.  due to the size a list box would not be feasible.  I have worked with drop-down list that have allowed more than one option to be selected.  I tried this:

topmostSubform.Page1.#subform[2].childInfoSubform.#subform[0].Table1.Row1.childInfoSubfor m.Mem_ChurchBdAux::ready:form - (JavaScript, client)

this.ui.choiceList.open = "multiSelect";

this.rawValue = "-1" ;

but this turns the drop-down list into a list-box.  I would truly appreciate anyone's help