When on the request list, click on any request you wish to link to another.
When clicking on three dot button, you would be given the option to link the request to an existing request.
You would then be able to search for the request you wish to link by subject name or by request ID.
Once linked you would find the linked request in the Child Request Tab.
If by any chance you added a request in error, don't panic. It can be unlinked when you click on the action button.