How to get QREXEC_* environment variables in systemd service?

I want to run:
/usr/lib/qubes/qrexec-client-vm --filter-escape-chars-stderr -- myVM user.Test /usr/lib/qubes/qfile-unpacker
Using systemd service but it fails because qfile-unpacker need to get QREXEC_REMOTE_DOMAIN environment variable to work:

And it seems that the QREXEC_* environment variables are somehow set from dom0 for started app in VM (passed with qrexec call?) and they are missing if the process is not started from qrexec.
Is there a way to somehow get these environment variables in systemd service?

Right now I manually set the variable and it works:
But it’s not a proper way because correct QREXEC_REMOTE_DOMAIN value may be changed in the future.

QREXEC_REMOTE_DOMAIN is used by qfile-unpacker only to set the directory name in QubesIncoming to which the files will be copied so the value of QREXEC_REMOTE_DOMAIN is not that important in this case.