Kubernetes cronjob are executed periodically. As they are a single object spawning jobs, if we want to run one periodically we can create manually a job from a cronjob :
kubectl -n my-namespace create job --from=cronjob/my-cronjob my-job-manual-1
We here create the job my-job-manual-1 from the cronjob my-cronjob inside my-namespace namespace.
Warning : the job and associated pods will not be reclaimed after some cronjob pass.