Don't know exactly why you want to use this for, but you'll have to PInvoke
this API (and some other)from credui.dll.
These API's are described in the Pltform SDK docs, check the wincred.h
header file for details on parameters and constants....

Following is a small sample to get you started...

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
struct _CREDUI_INFO
{
 public int cbSize;
 public IntPtr hwndParent;
 public string pszMessageText;
 public string pszCaptionText;
 public IntPtr hbmBanner;
}
class Program
{
 [DllImport("credui.dll",   CharSet=CharSet.Unicode)]
 internal static extern uint CredUIPromptForWindowsCredentials(ref
_CREDUI_INFO notUsedHere,
 int authError,
 ref uint  authPackage,
 IntPtr   InAuthBuffer,
 uint InAuthBufferSize,
 out IntPtr refOutAuthBuffer,
 out uint refOutAuthBufferSize,
 ref bool fSave,
 int flags);

 const int CREDUIWIN_AUTHPACKAGE_ONLY = 0x10;
  static void Main()
 {
      _CREDUI_INFO credui = new _CREDUI_INFO();
      credui.cbSize = Marshal.SizeOf(credui);
      credui.pszCaptionText = "Testje";
      credui.pszMessageText = "Message";
      uint authPackage = 0;
      IntPtr outCredBuffer;
      uint outCredSize;
      bool save = false;

      uint ret = CredUIPromptForWindowsCredentials(ref credui,
           0,
           ref authPackage,
           IntPtr.Zero,
           0,
           out outCredBuffer,
           out outCredSize,
           ref save,
           CREDUIWIN_AUTHPACKAGE_ONLY);

       if(ret != 0)
           // failed to load function...
       ...
       else
       // extract credentials from the buffer returned, using more
credui.dll API's .
       ...
       }
...

Willy.