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 link some existing Test case to some requirement directly from the Test issue screen, you just need to create a "tests" link between it and the requirement.
Make sure that the created link is "tests" (i.e. the Test covers the given issues)
Enter one, or more, issue key(s) of the requirement you wish to validate then click on "Link."
From the requirement issue screen
To link some existing Test case to some requirement directly from the requirement issue screen, you can do it using two different shortcuts as detailed in the first step below.
Using the "+" icon on the Test Coverage panel - Click the Edit button (at the top-left of the 'view issue' page) to open the Edit Issue dialog box.
Using the standard issue link creation dialog - Click on More and then Link
Like before, make sure that the created link is "tested by" (i.e. the requirement will be tested by the given issues.)
Enter one, or more, issue key(s) of the Test cases that validate your requirement 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.