#ifndef _NTRTL_H

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

#endif
View code on GitHub

// ntifs.h

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