This study is researching an experimental drug called mibavademab (also called "study drug"). The study is focused on patients with a condition called Functional Hypothalamic Amenorrhea (FHA). FHA is a condition where a woman stops menstruating because the brain is not sending the correct hormonal signals to the ovaries, which then are not able to cycle and ovulate properly. The aim of the study is to see how well the study drug helps the body make the hormones necessary for ovulation and reproduction, and to assess its safety. The study is looking at several other research questions, including: * Whether the drug helps bone health * What side effects may happen from taking the study drug * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)