In the requirement's right side vertical menu, click the test cases icon . You can either choose Link Test Case or New Test Case. When linking test cases, available test cases will be listed on the left side of the pop-up. Click on the test case(s), and then click Add to link them to the requirement.
New test cases created from the requirement will be automatically linked to that requirement version.
Users will need to have project permissions to view and edit in both Requirements and Test Coverage to perform this action. Depending on the settings of the user's project permissions, they may be able to view all the project requirements and all test coverage items, or just a subset of each.