<div asp-validation-summary="None" class="text-danger"></div>
<label asp-for="Username"></label>
<input asp-for="Username" class="form-control" />
<span asp-validation-for="Username" class="text-danger"></span>

<label asp-for="Password"></label>
<input asp-for="Password" class="form-control" />
<span asp-validation-for="Password" class="text-danger"></span>

[Required(ErrorMessage = "** Localized error message **")]
[Display(Name = "Username:", Prompt = "Username")]
public string Username { get; set; }

[Required(ErrorMessage = "** Another localized error message **")]
[DataType(DataType.Password)]
[Display(Name = "Password:", Prompt = "Password")]
public string Password { get; set; }

toastr.error(errorMessageOfInvalidLogin);