For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:agraul:branches:systemsmanagement:Uyuni:Master/openSUSE_Tumbleweed/home:agraul:branches:systemsmanagement:Uyuni:Master.repo
zypper refresh
zypper install spacewalk-setup
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:agraul:branches:systemsmanagement:Uyuni:Master/15.6/home:agraul:branches:systemsmanagement:Uyuni:Master.repo
zypper refresh
zypper install spacewalk-setup