You can link one or multiple test results to an existing issue.
Go to the test cases overview via Track > Test Runs. Open the sidebar by selecting the test case.
When clicking the Bug icon to the right of a test result in the sidebar.
You can either create a new issue or link to an existing issue. In this case, you select link to an existing issue.
A popup will appear with the test result in question already filled and an issue field where you can search the issue that needs to be linked.
Press Save, to save the changes you've made or Cancel to go back to the test cases overview page.