For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:agraul:branches:systemsmanagement:saltstack:products:next/openSUSE_Tumbleweed/home:agraul:branches:systemsmanagement:saltstack:products:next.repo
zypper refresh
zypper install salt
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:agraul:branches:systemsmanagement:saltstack:products:next/15.4/home:agraul:branches:systemsmanagement:saltstack:products:next.repo
zypper refresh
zypper install salt