login2.cs

[HttpPost]
public IActionResult Login([FromForm]LoginFormViewModel viewModel)
{
    var authClient = new AuthenticationApiClient("<your domain>.auth0.com");
    var authUrl = authClient.BuildAuthorizationUrl()
        .WithClient("<your client id>")
        .WithResponseType(AuthorizationResponseType.Code)
        .WithConnection("Username-Password-Authentication")
        .WithRedirectUrl("https://localhost:5001/home/callback")
        .WithScope("openid offline_access")
        .WithState("1234567890")
        .Build()
        .ToString();
    return Redirect(authUrl);
}