For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ecsos:messenger:mattermost/openSUSE_Tumbleweed/home:ecsos:messenger:mattermost.repo
zypper refresh
zypper install mattermost-desktop
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ecsos:messenger:mattermost/16.0/home:ecsos:messenger:mattermost.repo
zypper refresh
zypper install mattermost-desktop
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ecsos:messenger:mattermost/15.6/home:ecsos:messenger:mattermost.repo
zypper refresh
zypper install mattermost-desktop