Add Ref. No at starting of the custom artboard name

This would work:

function artboardRef() {
    var aDoc = app.activeDocument;
    for (i = 0; i < aDoc.artboards.length; i++) {
        var abName = aDoc.artboards[i].name;
        aDoc.artboards[i].name = (i + 1) + '_' + abName;
    }
}

artboardRef();