To find your course key, navigate to your course page via the left hand side navigation menu, then click the Add People button located in the upper right hand corner of the page.
In the pop-up modal, select Course Codes. You will be presented with a two access codes: one for students and TAs and one for instructors.
You can send the student code to any student or TA from your course to allow them to access the course in Mimir Classroom. For additional ways to add students, instructors and TAs to your course, read Adding Users to a Course.