MSAL 3x : AcquireTokenByUsernamePassword not working as documented : Microsoft.Identity.Client.MsalServiceException

Hi 

WPF, VS2017, MSAL 3x version.

I am trying an sample to test function AcquireTokenByUsernamePassword().  Following is the code example.I am getting error 

Error Acquiring Token:
Microsoft.Identity.Client.MsalServiceException: AADSTS7000218: The request body must contain the following parameter: 'client_assertion' or 'client_secret'.

  private async void MSALUsingScopeUserNamePasswordVersion3_Click(object sender, RoutedEventArgs e)
        {
            string[] scopes = new string[] { "<ResourceID XXXXXX>/user_impersonation" };
            string targetAPIUrl = string.Format("https://xxxxxxxxx.azurewebsites.net/api/TestFunction1");

            string ClientId = "3c278a32-0202-111c-8b03-xxxxxxxxxx";   
            string Tenant = "xxxxxx-7665-xxxx-8ce2-xxxxxxxxxxxx";


            IPublicClientApplication _clientApp;
            AuthenticationResult authResult = null;

            _clientApp = PublicClientApplicationBuilder.Create(ClientId)
               .WithAuthority(AzureCloudInstance.AzurePublic, Tenant)
               .Build();

            try
            {
                var securePassword = new SecureString();
                foreach (char c in "RealPassword123")        // you should fetch the password
                    securePassword.AppendChar(c);  // keystroke by keystroke

                authResult = await _clientApp.AcquireTokenByUsernamePassword(scopes, "ADUser@CompanyName.com", securePassword)
                    .ExecuteAsync();

                outputBox.Text = await GetHttpContentWithToken(targetAPIUrl, authResult.AccessToken);
            }
            catch (MsalException msalex)
            {
                outputBox.Text = $"Error Acquiring Token:{System.Environment.NewLine}{msalex}";
            }
        }

Please Advice.

Regards