Ensure that you have installed the Open Commit GitLab app on the organization that contains the repositories you want to have reviewed. You can check that you have installed the app by clicking here.
From the Open Commit Dashboard, select "Reviews" from the navigation bar. Make sure that you have selected the organization you want to view in the navigation bar in the drop-down menu.
Click the "Request Review" button on any of the pull requests you want to have reviewed on Open Commit. As soon as the project has been assigned, the status of the project will change to "Pending", and at that point you will no longer be able to cancel the review.