For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/openSUSE:infrastructure:software.opensuse.org/15.6/openSUSE:infrastructure:software.opensuse.org.repo
zypper refresh
zypper install software_staging_opensuse_org
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/openSUSE:infrastructure:software.opensuse.org/15.5/openSUSE:infrastructure:software.opensuse.org.repo
zypper refresh
zypper install software_staging_opensuse_org