Где onclick
, просто измените его на onChange="javascript://whatever function you want()"
, например
@Html.DropDownListFor(model => Tlitem.JobType, (SelectList)newSelectList, new { @class = "form-control js-select js-noFilter hidden", size = "2", @value = 0, Id = "JobType" + t ,@onchange="javascript:onChangeFunction()";