Implementing a Simple Registration Code Check in ASP.NET MVC

ExternalLoginConfirmation.cs.html

<li class="name">
    @Html.LabelFor(m => m.UserName)
    @Html.TextBoxFor(m => m.UserName)
    @Html.ValidationMessageFor(m => m.UserName)
</li>
<li class="name">
    @Html.LabelFor(m => m.RegistrationCode)
    @Html.TextBoxFor(m => m.RegistrationCode)
    @Html.ValidationMessageFor(m => m.RegistrationCode)
</li>

Register.cs.html

<li>
    @Html.LabelFor(m => m.UserName)
    @Html.TextBoxFor(m => m.UserName)
</li>
<li>
    @Html.LabelFor(m => m.RegistrationCode)
    @Html.TextBoxFor(m => m.RegistrationCode)
</li>
<li>
    @Html.LabelFor(m => m.Password)
    @Html.PasswordFor(m => m.Password)
</li>

RegisterExternalLoginModel.cs

public class RegisterExternalLoginModel
{
    [Required]
    [Display(Name = "User name")]
    public string UserName { get; set; }

    [Required]
    [ValidRegistrationCode]
    [Display(Name = "Registration Code")]
    public string RegistrationCode { get; set; }

    public string ExternalLoginData { get; set; }
}

RegisterModel.cs

public class RegisterModel
{
    [Required]
    [Display(Name = "User name")]
    public string UserName { get; set; }

    [Required]
    [ValidRegistrationCode]
    [Display(Name = "Registration Code")]
    public string RegistrationCode { get; set; }

    [Required]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
}

ValidRegistrationCodeAttribute.cs

public class ValidRegistrationCodeAttribute : ValidationAttribute
{

    public ValidRegistrationCodeAttribute()
    {
        this.ErrorMessage = "Invalid registration code.";
    }

    public override bool IsValid(object value)
    {
        if (value == null) return false;
        // In this example, I am storing my registration code in the web.conf file. 
        // You can store the code however you like including web.config or a database.
        var validCode = ConfigurationManager.AppSettings["RegistrationCode"];
        return validCode.Equals(value.ToString());
    }

}

how to put registration code in the web config sir?