In the requirement's right side vertical menu, click the task icon and then click New/Link. Select + New Task in the dropdown to create a new task. Enter a task name in the text area and click Save. You can then click on the task ID to view the details and edit. Alternatively, an existing task can be linked to the requirement by selecting that option in the New/Link button dropdown.
Users will need to have project permissions to view and edit in both Requirements and Tasks 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 project tasks, or just a subset of each. Users added to a specific requirement as a co-owner/contributor/reviewer, and users added to a specific task as a watcher will be able to view them.