Where to remove domain from username? ASP.NET MVC 4

Question

This is an intranet application written in MVC 4 which requires user login. Where is the best place to remove the domain from the username entered by the user? (Username.Name.Split('\')[1];). This is incase the user enters the domain.

View:

@Html.ValidationSummary(true, "Login failed.")
@using (Html.BeginForm())
{
    <div style="width: 400px">
        <fieldset>
            <legend>Account Information</legend>
            <div class="editor-label">
                @Html.LabelFor(m => m.Username)
            </div>
            <div class="editor-field">
                @Html.TextBoxFor(m => m.Username)
                @Html.ValidationMessageFor(m => m.Username)
            </div>
            ...

Login Model:

[Required]
[Display(Name = "Username")]
public string Username { get; set; }
....

Controller:

[HttpPost]
public ActionResult Login(LoginModel model, string returnUrl)
{
    if (ModelState.IsValid)
        {
        if (Membership.ValidateUser(model.Username, model.Password))
            {
                //set authentication cookie
                ...            

Answer 1

If you only want the username and not the domain why not validate as early as possible, e.g. don't all \ in username. As always have server validation to back this up in Login action like you suggest, e.g.

if(model.UserName.Contains('\')  
         model.UserName = model.Username.Name.Split('\')[1]; 

If it is always the same domain, put the domain name beside the text box to make it clear it is not being asked for.