This trial is testing a medication called bimekizumab to see if it is safe and effective for people with a type of chronic back and joint inflammation called axial spondyloarthritis, which includes ankylosing spondylitis. The medication works by blocking proteins that cause inflammation, helping to reduce pain and swelling. Bimekizumab has shown promising results in previous studies for conditions like psoriatic arthritis and ankylosing spondylitis.