This study is testing a new combination of medicines, **petosemtamab** and **pembrolizumab**, to see if they work better together than **pembrolizumab** alone for treating **head and neck cancer** that has come back or spread to other parts of the body. **Pembrolizumab** is a medicine that helps the immune system fight cancer by targeting a protein called **PD-L1**. **Petosemtamab** is being tested to see if it can help **pembrolizumab** work even better. Patients must be 18 years or older, have specific types of head and neck cancer, and meet other health criteria. They should not have had certain previous cancer treatments. The study is **open-label**, meaning both patients and doctors know what treatment is being given, and it's **randomized**, meaning patients are randomly assigned to either the new combination or just pembrolizumab. **Key Points:** - The study is for adults with specific types of head and neck cancer. - Participants will be randomly chosen to receive either the new treatment combination or pembrolizumab alone. - Certain previous treatments and health conditions may exclude participation. This study aims to find a better treatment option for people with recurrent or metastatic head and neck cancer.