For openSUSE Leap 15.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Rotkraut/openSUSE_Leap_15.2/home:Rotkraut.repo
zypper refresh
zypper install josm
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Rotkraut/15.6/home:Rotkraut.repo
zypper refresh
zypper install josm
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Rotkraut/15.5/home:Rotkraut.repo
zypper refresh
zypper install josm
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Rotkraut/15.4/home:Rotkraut.repo
zypper refresh
zypper install josm