Assume this model:
Public Class Detail
...
I make this putting this on my view
@Html.DropDownListFor(m => m.BenefNamePos, Model.Options, new { onchange = "changePosition(this);", @class="form-control", data_val_number = "This is my custom message" })
Or you can simply do this.
@Html.ValidationMessageFor(m => m.PercentChange, "Custom Message: Input value must be a number"), new { @style = "display:none" })
Hope this helps.