This feature (parent/child/simple ticket relations) is currently not available, but will be added in a later version (6.1), which is still a long way off.
For now you could make use of a custom field or add the number to a note. Not as good as a link, but you can copy the ID and use it with the 'Go to ticket' action from the helpdesk menu (Ctrl + click or middle mouse click on this link to open in a new tab).
We'll be adding a new feature in one of our major upcoming versions (probably 6.0.110, which will be a few more months at least) which will automagically convert ticket IDs in notes (format: #123) to a link.