For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:proxy/openSUSE_Factory/server:proxy.repo
zypper refresh
zypper install dansguardian
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:proxy/15.6/server:proxy.repo
zypper refresh
zypper install dansguardian
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:proxy/15.5/server:proxy.repo
zypper refresh
zypper install dansguardian