When you say you enabled 'self enrolment' do you mean you enabled it for the course or you allowed users to create accounts on your site? If you're new to MoodleCloud there are videos and very simple text explanations in our Admin Basics MOOC which demonstrate different ways of getting users on your site. As for what URL to send, when you go into your course, you look at the link in the browser and you can copy and past that and send to them. But they will need to have an account on the site before they can self enrol into the course.
See this playlist of 26 videos