Install using μpkg (see the latest release for instructions) and then symlink the systemd unit from your user systemd dir:
upkg install orbit-online/socket-credential-cache@<VERSION>
ln -s ../../../.local/lib/upkg/.upkg/socket-credential-cache/socket-credential-cache@.service $HOME/.config/systemd/user/socket-credential-cache@.service
systemctl --user daemon-reload