@Html.TextBoxFor(m => m.FirstName)
@{
   var emailSubject = the value of @Html.TextboxFor( m => m.FirstName) + "Thank you for Contancting Us"
 }
@Html.Hidden(ThankyouNote, emailSubject)

@Html.TextBoxFor(m => m.FirstName)
@{
   var emailSubject = Model.Firstname + "Thank you for Contancting Us"
 }
@Html.Hidden(ThankyouNote, emailSubject)