For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ighvh:devel-kernels/15.5/home:ighvh:devel-kernels.repo
zypper refresh
zypper install kernel-source-0504rtai
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ighvh:devel-kernels/15.4/home:ighvh:devel-kernels.repo
zypper refresh
zypper install kernel-source-0504rtai
For 15.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ighvh:devel-kernels/15.2/home:ighvh:devel-kernels.repo
zypper refresh
zypper install kernel-source-0504rtai