For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:otrs/openSUSE_Factory/network:otrs.repo
zypper refresh
zypper install otrs
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:otrs/15.4/network:otrs.repo
zypper refresh
zypper install otrs
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:otrs/15.3/network:otrs.repo
zypper refresh
zypper install otrs