For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ithod:signal/openSUSE_Tumbleweed/home:ithod:signal.repo
zypper refresh
zypper install signal-desktop
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ithod:signal/15.6/home:ithod:signal.repo
zypper refresh
zypper install signal-desktop
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ithod:signal/15.5/home:ithod:signal.repo
zypper refresh
zypper install signal-desktop