For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:Factory:git-workflow:staging:regularhunter:weechat:1/openSUSE_Tumbleweed/devel:Factory:git-workflow:staging:regularhunter:weechat:1.repo
zypper refresh
zypper install weechat
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:Factory:git-workflow:staging:regularhunter:weechat:1/15.6/devel:Factory:git-workflow:staging:regularhunter:weechat:1.repo
zypper refresh
zypper install weechat