Find the Jira app in the right-side panel and click Link an existing issue (1) to get started.
The app shows a selection of the five most recently used issues. If you do not see the issue that you are looking for use the issue search (1) to find the issue. You can search by issue key, e.g. ENG-25, or by text.
When you have found the issue that you would like to link simply click on it to link (2).
A confirmation message will indicate that the link has been established successfully. Click Done to return to the app home screen where you can now see the linked Jira issue (1) and an internal note (2) to record the event.