flatpak from home:mkrwc:zigzag:15.3-backports project
Select Your Operating System
Add repository and install manually
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mkrwc:zigzag:15.3-backports/15.3/home:mkrwc:zigzag:15.3-backports.repo zypper refresh zypper install flatpak