Cannot find a way to add a placeholder for an MVC 5 DropDownListFor

Try this:

@Html.DropDownListFor(model => model.CustomerID, 
    new SelectList(ViewBag.Customers, "CustomerID", "Email"), 
    "-- Please Select --", 
    new { htmlAttributes = new { @class = "form-control" } })

3rd overload can be the "placeholder" (optionLabel).

A select box doesnt have a "placeholder" like text inputs do.