Sorry Pär, there isn't currently a webservice that does that. See https://github.com/mdjnelson/moodle-mod_customcert/blob/MOODLE_37_STABLE/classes/external.php for a list of available webservices. Feel free to add one and create a pull request.
What if I was doing some testing and deleted the certificates? Now I want them to be issued and it is not doing so? Any thoughts?