Sorry - to be clear - I was referring to activity completion - not course completion.
But if course completion is achieved through activity completion then it can be useful.
If you are new to Moodle and/or are not familiar with APIs it will take a little learning.
Here is the moodle documentation: https://docs.moodle.org/311/en/Using_web_services
This video on setting up a web service might help
See also https://docs.moodle.org/dev/Web_service_API_functions - the relevant one for this is ore_completion_override_activity_completion_status