I'm new to Moodle and need to create a trigger

Hi, We've deployed Moodle and I have no idea what I'm doing

I'd like our course(s) to invoke a web service to an external app once a student has completed it (trigger??). Perhaps I need to build a client (web service) app and have Moodle pass it parameters and invoke it.

What's the accepted "Moodle" way I should do this?

Thanks, Chris