For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:matthewtaylor/openSUSE_Tumbleweed/home:matthewtaylor.repo
zypper refresh
zypper install qemu-guest-agent
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:matthewtaylor/openSUSE_Slowroll/home:matthewtaylor.repo
zypper refresh
zypper install qemu-guest-agent
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:matthewtaylor/15.6/home:matthewtaylor.repo
zypper refresh
zypper install qemu-guest-agent