• There is no such action, the API UI shows you all the actions that exist so making up new ones doesn't do anything..
• There is also no concept of "run it now" in general in k8s. What you want to do is "create a new (unrelated) job resource that maybe happens to have a very similar looking spec to an existing cronjob resource"
• You're just dealing with regular k8s objects, use the regular k8s api and one of the many clients that I'm sure exist for it in your language. There's no reason to involve Rancher.