Traceability link can be set between items from its Detail view.
For example, you want to set traceability from a defect to a user story.
1. Navigate to your project either from Quick Links widget, or click Projects and double-click the project row from the list. Roll over the project name in the breadcrumb, and click Cards > Defects.
2. Double-click the defect you want to open. Click the Traceability section and then the Add icon on the right.
3.Now, select the workspace in which the user story is created, and item type as User Stories. You may enter text as a search string, which you know is a part of the user story title. The Searched Items pane on the right lists down the searched items.
3.From the Searched Items panel, drag the required item to the Traced items section on the left. Click Save.

When you return to the Traceability tab, the tab displays the relationship set. For a listing of the traceability links, click the List View button on the right. In this example, you have set relationship from a defect to a user story, so the user story is listed in the Upward Traced Items panel. If you have set a relationship from an issue to this defect, here the issue will be listed in the Downward Traced Items panel.