For openSUSE Leap 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/windows:mingw:win64/openSUSE_Leap_15.6/windows:mingw:win64.repo
zypper refresh
zypper install mingw64-mono
For openSUSE Leap 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/windows:mingw:win64/openSUSE_Leap_15.5/windows:mingw:win64.repo
zypper refresh
zypper install mingw64-mono
For openSUSE Leap 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/windows:mingw:win64/openSUSE_Leap_15.4/windows:mingw:win64.repo
zypper refresh
zypper install mingw64-mono