For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:j-engel:samba-stable/15.6/home:j-engel:samba-stable.repo
zypper refresh
zypper install samba-ad-dc
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:j-engel:samba-stable/15.5/home:j-engel:samba-stable.repo
zypper refresh
zypper install samba-ad-dc
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:j-engel:samba-stable/15.4/home:j-engel:samba-stable.repo
zypper refresh
zypper install samba-ad-dc