For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:polyconvex/openSUSE_Tumbleweed/home:polyconvex.repo
zypper refresh
zypper install flatpak
For openSUSE Leap 15.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:polyconvex/openSUSE_Leap_15.2/home:polyconvex.repo
zypper refresh
zypper install flatpak
For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:polyconvex/openSUSE_Factory/home:polyconvex.repo
zypper refresh
zypper install flatpak