NtOpenTransactionManager - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTTMAPI_H
#if (PHNT_VERSION >= PHNT_VISTA)

NTSYSCALLAPI
NTSTATUS
NTAPI
NtOpenTransactionManager(
    _Out_ PHANDLE TmHandle,
    _In_ ACCESS_MASK DesiredAccess,
    _In_opt_ POBJECT_ATTRIBUTES ObjectAttributes,
    _In_opt_ PUNICODE_STRING LogFileName,
    _In_opt_ LPGUID TmIdentity,
    _In_opt_ ULONG OpenOptions
    );

#endif
#endif

View code on GitHub
#ifndef _NTZWAPI_H

NTSYSCALLAPI
NTSTATUS
NTAPI
ZwOpenTransactionManager(
    _Out_ PHANDLE TmHandle,
    _In_ ACCESS_MASK DesiredAccess,
    _In_opt_ POBJECT_ATTRIBUTES ObjectAttributes,
    _In_opt_ PUNICODE_STRING LogFileName,
    _In_opt_ LPGUID TmIdentity,
    _In_opt_ ULONG OpenOptions
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit here and here.