You will find the CronJobresource in the batch/v1beta1 API group. To create a new CronJob resource, you'll need a POST call to the /apis/batch/v1beta1/namespaces/{namespace}/cronjobs URL.
A respective HTTP request might look something like this:
POST /apis/batch/v1beta1/namespaces/default/cronjobs HTTP/1.1
Content-Type: application/json
Content-Length: ...
Authorization: ...
[other headers]
{
"metadata": {
"name": "some-cron"
},
"spec": {
...
}
}