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Step 1a: 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.
Step 1b: Using the Jira native issue link creation dialog - Click on More and then Link
Step 2: Make sure that the created link is "tested by" (i.e. the requirement will be tested by the given issues)
Step 3: Enter one, or more, issue key(s) of the Test cases that validate your requirement.
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