$ k3d cluster create test -a 1
...

$ k3d node create testnode2 --role agent --cluster test --k3s-arg '--node-label=foo=bar'
...

$ docker inspect k3d-testnode2-0
...
"Cmd": [
                "agent",
                "--node-label=foo=bar"
            ],
...

$ k3d node create testnode3 --role agent --cluster test --k3s-arg '--node-label=spam=eggs'
...

$ kubectl get nodes --show-labels                                                                                      
NAME                STATUS   ROLES                  AGE     VERSION        LABELS
k3d-testnode2-0     Ready    <none>                 4m11s   v1.22.7+k3s1   beta.kubernetes.io/arch=amd64,beta.kubernetes.io/instance-type=k3s,beta.kubernetes.io/os=linux,foo=bar,kubernetes.io/arch=amd64,kubernetes.io/hostname=k3d-testnode2-0,kubernetes.io/os=linux,node.kubernetes.io/instance-type=k3s
k3d-testnode3-0     Ready    <none>                 3m44s   v1.22.7+k3s1   beta.kubernetes.io/arch=amd64,beta.kubernetes.io/instance-type=k3s,beta.kubernetes.io/os=linux,foo=bar,kubernetes.io/arch=amd64,kubernetes.io/hostname=k3d-testnode3-0,kubernetes.io/os=linux,node.kubernetes.io/instance-type=k3s,spam=eggs
k3d-test-agent-0    Ready    <none>                 8m47s   v1.22.7+k3s1   beta.kubernetes.io/arch=amd64,beta.kubernetes.io/instance-type=k3s,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k3d-test-agent-0,kubernetes.io/os=linux,node.kubernetes.io/instance-type=k3s
k3d-test-server-0   Ready    control-plane,master   8m52s   v1.22.7+k3s1   beta.kubernetes.io/arch=amd64,beta.kubernetes.io/instance-type=k3s,beta.kubernetes.io/os=linux,kubernetes.io/arch=amd64,kubernetes.io/hostname=k3d-test-server-0,kubernetes.io/os=linux,node-role.kubernetes.io/control-plane=true,node-role.kubernetes.io/master=true,node.kubernetes.io/instance-type=k3s