For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:rhabacker:branches:windows:mingw:win64:KF6/15.6/home:rhabacker:branches:windows:mingw:win64:KF6.repo
zypper refresh
zypper install mingw64-kf6-filesystem
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:rhabacker:branches:windows:mingw:win64:KF6/15.5/home:rhabacker:branches:windows:mingw:win64:KF6.repo
zypper refresh
zypper install mingw64-kf6-filesystem