On the change details page, under Planning > Associations, you can associate the requests caused by the change, the requests that this change initiated, and problems.
On the displayed page, click the Plus icon, and from the displayed window, select the requests or problems, to be associated, under the respective blocks.
While associating requests with a change, the Requests that Initiated Change option will allow you to associate requests that are already associated with other changes. However, requests that are part of Requests Caused by Change association cannot be associated.
The following screenshot shows all the associations for the planning stage for a change request.