Instructors will often create a demo student account for accessing a course as a student. Such an account can be used with Respondus Monitor without impacting the institution's license seat count, or student purchase of an additional seat. Within the settings area for Respondus Monitor, select “Advanced Settings” and enter a username for the demo student. Note that the username for the demo student cannot be changed once used with a Respondus Monitor session.

Keep in mind that you cannot log in as an instructor and then impersonate this student role. This will result in no video showing up afterwards in the "class results" section of the LockDown Browser Dashboard tool.
Rather, you must use LockDown Browser to directly log in with that demo student's credentials.