For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/16.0/X11:xfce.repo
zypper refresh
zypper install package-update-indicator
For 15.7 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/15.7/X11:xfce.repo
zypper refresh
zypper install package-update-indicator
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce/15.6/X11:xfce.repo
zypper refresh
zypper install package-update-indicator