David,
Forums are an activity and activities only live inside courses. So, the usual and easiest way to do this is set up a course just for this purpose, sort of a "home course" for the cohort.
You could set up one course per cohort if you want, or you could set up one course for multiple cohorts, and then using the Cohort to Group plugin (https://moodle.org/mod/forum/discuss.php?d=417345) you can set the course to Separate Groups so all the cohorts/groups could work in one forum but not see each other.
Forums are an activity and activities only live inside courses. So, the usual and easiest way to do this is set up a course just for this purpose, sort of a "home course" for the cohort.
You could set up one course per cohort if you want, or you could set up one course for multiple cohorts, and then using the Cohort to Group plugin (https://moodle.org/mod/forum/discuss.php?d=417345) you can set the course to Separate Groups so all the cohorts/groups could work in one forum but not see each other.