How to advertise the grad course that you teach?
First, make up an attractive one-page flyer that details what your course is about and when it's offered, and that it's STEM certified (sounds important, but I've never heard of it).

Then, contact departments at your university and at others in the consortium that may be interested in your course and ask if they would send out your flyer advertising your course to their graduate departmental mailing list. 

You might also note there needs to be a minimum enrollment (encourage people on the fence to sign up).

Where I'm at, this would usually be an advisor's job, or maybe a program coordinator, but if you know a sympathetic professor in the department you're contacting, they could also reach out to whoever manages the mailing list.