In the documentation Installation — restic 0.11.0 documentation
Windows it says
On Windows, put the restic.exe binary into %SystemRoot%\System32 to use restic in scripts without the need for absolute paths to the binary. This requires administrator rights.
That is something I would never do or recommend. The idea of putting any part of an application into the directory structure of another will cause problems with administration and especially app/OS upgrades. After a windows update you probably will have fix it because restic.exe will be in the C:\Windows.old folder and will be completely gone in the default of10 days