private static extern Boolean CredPackAuthenticationBuffer(

    Public Shared Function CredPackAuthenticationBuffer(ByVal dwFlags As UInt32, ByVal pszUserName As String, ByVal pszPassword As String, ByVal pPackedCredentials As IntPtr, ByRef pcbPackedCredentials As UInt32) As <MarshalAs(UnmanagedType.Bool)> Boolean

    CredPackAuthenticationBuffer(0, username, password, inCredBuffer, ref inCredSize);

    ''' <param name="pvInAuthBuffer">An authentication buffer with which to default prompted values (see <see cref="CredPackAuthenticationBuffer" />).</param>

      (IntPtr)0,  //You can force that a specific username is shown in the dialog. Create it with 'CredPackAuthenticationBuffer()'. Then, the buffer goes here...