For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:asdhio/openSUSE_Tumbleweed/home:asdhio.repo
zypper refresh
zypper install plank
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:asdhio/16.0/home:asdhio.repo
zypper refresh
zypper install plank
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:asdhio/15.6/home:asdhio.repo
zypper refresh
zypper install plank