This study focuses on people who have chronic rhinosinusitis (inflammation or swelling of the tissues lining the sinuses and the nose). The study will evaluate the effectiveness, safety, and tolerability of an experimental drug called itepekimab in adults with inadequately controlled chronic rhinosinusitis with nasal polyps. Researchers want to determine whether adding itepekimab to a nasal steroid spray is safe and can help improve symptoms. The study will test 2 different doses of itepekimab as add-on therapy to nasal steroid spray, compared to placebo. Specifically, researchers will be looking to see whether itepekimab decreases nasal polyp size, improves nasal congestion, and reduces sinus inflammation, compared to the standard-of-care treatment. Participants will be randomly assigned to either the itepekimab and standard-of-care (consisting of the mometasone furoate nasal spray) group, or the placebo and standard-of-care group. Itepekimab is a human monoclonal antibody (a type of protein) that blocks the activity of interleukin-33 (IL-33) in the body (interleukins are proteins that stimulate immune response and can cause inflammation). IL-33 may play a role in chronic rhinosinusitis with nasal polyps, and blocking the effects of IL-33 may improve chronic rhinosinusitis with nasal polyp symptoms.