#ifndef _NTRTL_H

NTSYSAPI
NTSTATUS
NTAPI
RtlCustomCPToUnicodeN(
    _In_ PCPTABLEINFO CustomCP,
    _Out_writes_bytes_to_(MaxBytesInUnicodeString, *BytesInUnicodeString) PWCH UnicodeString,
    _In_ ULONG MaxBytesInUnicodeString,
    _Out_opt_ PULONG BytesInUnicodeString,
    _In_reads_bytes_(BytesInCustomCPString) PCH CustomCPString,
    _In_ ULONG BytesInCustomCPString
    );

#endif
View code on GitHub

// ntifs.h

NTSYSAPI NTSTATUS RtlCustomCPToUnicodeN(
  PCPTABLEINFO CustomCP,
  PWCH         UnicodeString,
  ULONG        MaxBytesInUnicodeString,
  PULONG       BytesInUnicodeString,
  PCH          CustomCPString,
  ULONG        BytesInCustomCPString
);
View the official Windows Driver Kit DDI reference