For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:d1vanov:sstp/openSUSE_Tumbleweed/home:d1vanov:sstp.repo
zypper refresh
zypper install NetworkManager-sstp
For openSUSE Factory PowerPC run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:d1vanov:sstp/openSUSE_Factory_PowerPC/home:d1vanov:sstp.repo
zypper refresh
zypper install NetworkManager-sstp
For openSUSE Factory ARM run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:d1vanov:sstp/openSUSE_Factory_ARM/home:d1vanov:sstp.repo
zypper refresh
zypper install NetworkManager-sstp