Click on the requirement to view the details. On the right side vertical menu, click the info button to expand the info panel, and then click the add button in the Tags field. Choose the tags to add from the dropdown. Tags can be created and made available in the Settings tab by a project owner or members with access to configure project settings. These tags will be listed in the dropdown menu when the add button is clicked.
If you have access to configure project settings, you may add a new tag at the top of the pop-up where it reads Add a new tag, then select Save to add it to the list of tags for the project.
A user must have view and edit permissions in requirements and access to configure project settings to add a tag. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset of them. Users added to a specific requirement as a contributor or reviewer will be able to view it.