Login in Moodle web as admin and go to  Site administration > Plugins > Enrolments > Self enrolment . In there, enable "Show hint". Now create a course, enter it, click the gear icon and click "More...". In there, select the "Users" tab and click in "Enrolment methods". Enable self enrolment. Click the gear icon in self enrolment and add a password. Now login in the app and go to search courses (top right icon in course overview). Search the course you created before and click on it. Check that you see the "Self enrol" button. Click the self enrol button and check that a modal to input the password is displayed. Check that entering a wrong password shows you a hint. Check that entering the valid password self enrols you in the course.