For example, if your webapp (webapp-c58766496-vz7tc) wants to connect to mysql service, since the service is in another namespace, you would need to use webapp-service.svc.payroll.cluster.local.
controlplane ~ ➜ k get po -owide -A
NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
default webapp-c58766496-vz7tc 1/1 Running 0 7s 172.17.0.14 controlplane <none> <none>
payroll mysql 1/1 Running 0 13m 172.17.0.10 controlplane <none> <none>
controlplane ~ ➜ k get svc -owide
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
kubernetes ClusterIP 172.20.0.1 <none> 443/TCP 36m <none>
webapp-service NodePort 172.20.110.197 <none> 8080:30082/TCP 16m name=webapp
controlplane ~ ➜ k get nodes -owide
NAME STATUS ROLES AGE VERSION INTERNAL-IP EXTERNAL-IP OS-IMAGE KERNEL-VERSION CONTAINER-RUNTIME
controlplane Ready control-plane 36m v1.33.0 192.168.56.151 <none> Ubuntu 22.04.5 LTS 5.15.0-1083-gcp containerd://1.6.26