Covering some requirement issue with an existing Test can either be done from the Test issue screen or from the requirement issue screen.
Requirement and Tests
How to Link from the Test issue screen
To associate an existing Test/Test Set with the Requirement:
Click Link.
The Jira Issue link dialog will appear. Make sure to select the correct link type (e.g., "is tested by") along with the issue key of the Test(s)/Test Set(s).
Enter one, or more, issue key(s) of the requirement you wish to validate then click on "Link."
From the requirement issue screen
To associate an existing Test/Test Set with the Requirement:
Click Link.
The Jira Issue link dialog will appear. Make sure to select the correct link type (e.g., "is tested by") along with the issue key of the Test(s)/Test Set(s).
Enter one, or more, issue key(s) of the requirement you wish to validate then click on "Link."
You may also specify Test Set issue keys; in this case, the link will be created between the requirement and the Test Set.
When you do this, you'll have a dynamic coverage of the requirement based on the Tests belonging to the Test Set. That means that you can then add or remove Tests to/from the Test Set and they will be covering the requirement or not depending on being or not on the Test Set.