public class CredentialDialog
  {
    private const int MaxBufferLength = 100;

    public string Caption { get; set; }
    public string Message { get; set; }

    public string Username { get; private set; }
    public SecureString Password { get; private set; }

    public unsafe DialogResult ShowDialog(IWin32Window owner = null)
    {
      var info = new NativeMethods.CredentialUiInfo
      {
        pszCaptionText = this.Caption,
        pszMessageText = this.Message,
      };

      info.cbSize = Marshal.SizeOf(info);

      if (owner != null)
        info.hwndParent = owner.Handle;

      uint authPackage = 0;
      var authError = 0;
      var save = false;

      var result = NativeMethods.CredUIPromptForWindowsCredentials(ref info,
                                                                   authError,
                                                                   ref authPackage,
                                                                   IntPtr.Zero,
                                                                   0,
                                                                   out var outCredBuffer,
                                                                   out var outCredSize,
                                                                   ref save,
                                                                   0x1);

      var usernameBuffer = new StringBuilder(MaxBufferLength);
      var domainBuffer = new StringBuilder(MaxBufferLength);

      var usernameLength = MaxBufferLength;
      var passwordLength = MaxBufferLength;
      var domainLength = MaxBufferLength;

      var pwPointer = Marshal.AllocHGlobal(MaxBufferLength);

      NativeMethods.CredUnPackAuthenticationBuffer(0,
                                                   outCredBuffer,
                                                   outCredSize,
                                                   usernameBuffer,
                                                   ref usernameLength,
                                                   domainBuffer,
                                                   ref domainLength,
                                                   pwPointer,
                                                   ref passwordLength);

      Marshal.FreeCoTaskMem(outCredBuffer);

      this.Username = usernameBuffer.ToString();
      this.Password = new SecureString((char*)pwPointer, passwordLength);

      return result == (int)NativeMethods.CredentialDialogResult.Ok
               ? DialogResult.OK
               : DialogResult.Cancel;
    }

    private static class NativeMethods
    {

      public enum CredentialDialogResult
      {
        Ok = 0,
        Cancel = 1223
      }

      [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
      public struct CredentialUiInfo
      {
        public int cbSize;
        public IntPtr hwndParent;
        public string pszMessageText;
        public string pszCaptionText;
        public IntPtr hbmBanner;
      }

      [DllImport("credui.dll", CharSet = CharSet.Auto)]
      public static extern bool CredUnPackAuthenticationBuffer(int dwFlags,
                                                               IntPtr pAuthBuffer,
                                                               uint cbAuthBuffer,
                                                               StringBuilder pszUserName,
                                                               ref int pcchMaxUserName,
                                                               StringBuilder pszDomainName,
                                                               ref int pcchMaxDomainame,
                                                               IntPtr pszPassword,
                                                               ref int pcchMaxPassword);

      [DllImport("credui.dll", CharSet = CharSet.Auto)]
      public static extern int CredUIPromptForWindowsCredentials(ref CredentialUiInfo notUsedHere,
                                                                 int authError,
                                                                 ref uint authPackage,
                                                                 IntPtr InAuthBuffer,
                                                                 uint InAuthBufferSize,
                                                                 out IntPtr refOutAuthBuffer,
                                                                 out uint refOutAuthBufferSize,
                                                                 ref bool fSave,
                                                                 int flags);
    }
  }