Mapping Server Network Drive

Using a QNAP server for media.
Windows is mapped R: for server. The Mac is connected via TB3 to the QNAP. How do I map the Mac to the Windows system? I tried R:/ but that did not work.
Windows is mapped R: for server. The Mac is connected via TB3 to the QNAP. How do I map the Mac to the Windows system? I tried R:/ but that did not work.