For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/openSUSE:infrastructure:monitoring/openSUSE_Tumbleweed/openSUSE:infrastructure:monitoring.repo
zypper refresh
zypper install golang-github-prometheus-alertmanager
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/openSUSE:infrastructure:monitoring/15.6/openSUSE:infrastructure:monitoring.repo
zypper refresh
zypper install golang-github-prometheus-alertmanager