public sealed class UserModel
{
    [Key]
    [Required]
    [StringLength(20, MinimumLength = 4)]
    public string Username { get; set; }

    [Required]
    [EmailAddress]
    [DataType(DataType.EmailAddress)]
    public string Email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [StringLength(20, MinimumLength = 4)]
    public string Password { get; set; }

    public bool IsValid { get; set; }
}

public class UserViewModel {
    [Required]
    [StringLength(20, MinimumLength = 4)]
    public string Username { get; set; }

    [Required]
    [EmailAddress]
    [DataType(DataType.EmailAddress)]
    public string Email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [StringLength(20, MinimumLength = 4)]
    public string Password { get; set; }
}

public ActionResult Post([FromBody] UserViewModel viewModel) {
    if(ModelState.IsValid) {
        //create new model
        var model = new UserModel {
            Username = viewModel.Username,
            Email = viewModel.Email,
            Password = viewModel.Password
        };

        //or model retrieved from data storage

        //...some other code...

        model.IsValid = true; //only the server can modify this value

        //...
    }

    //...
}