Click on the requirement to view the details. The requirement owner can click the Draft status dropdown and select Send for Review from the menu. On confirmation, a due date may be set as a request for reviewers to provide their response. Click the Yes button to send the requirement version for review. You can only send a requirement for review if it has at least one reviewer added.
Users will need to be the requirement owner, and have project permissions to view and edit the requirement to send a requirement for review. 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. The same is true for viewing the list of project members when adding project contributors and reviewers. Users added to a specific requirement as a contributor or reviewer will be able to view it.