CreateIoCompletionPort

[DllImport("Kernel32.dll")][return: MarshalAs(UnmanagedType.SafeHandle)]
public static extern SafeFileHandle CreateIoCompletionPort(
   IntPtr FileHandle,
   IntPtr ExistingCompletionPort,
   UIntPtr CompletionKey,
   uint NumberOfConcurrentThreads
);

Microsoft documentation: Link