Multi-Select Combo Box Functionality Not Working

I have created attributes on the attributes form below

And these values should be shown as multi-select in Sales Order Line in a user defined field, code is below for getting those values, but in the field it only shows combo box (one value selection).

public class SOOrderEntry_Extension : PXGraphExtension<SOOrderEntry>
  {
    #region Event Handlers
    protected void SOLine_UsrAccessories_FieldSelecting(PXCache cache, PXFieldSelectingEventArgs e)
    {

     List<string> allowedValues = new List<string>();
     List<string> allowedLabels = new List<string>();

            foreach (CSAttributeDetail objCSAttributeDetail in PXSelect<CSAttributeDetail,
                                                      Where<CSAttributeDetail.attributeID, Equal<Required<CSAttributeDetail.attributeID>>>>.Select(Base, "ACCITEMS"))


            {
                allowedValues.Add(objCSAttributeDetail.ValueID);
                allowedLabels.Add(objCSAttributeDetail.Description);
            }

            e.ReturnState = PXStringState.CreateInstance(e.ReturnState, 10, true, typeof(SOLineExt.usrAccessories).Name, false, -1, string.Empty, allowedValues.ToArray(), allowedLabels.ToArray(), false, null);



    }