According to the PMBOK Guide, the project manager should be identified prior to beginning the Planning processes and is named and authorized in the project charter.

When a product owner requests the addition of a new mandatory feature during a sprint, it can disrupt the planned work and cause frustration within the team. The best course of action is to engage the product owner in prioritizing the product backlog with the project team. This collaborative approach ensures that the team understands the value and urgency of the new feature, aligns on the priorities, and adjusts their work plan accordingly. It also helps in managing the team's workload and expectations, preventing overcommitment and potential burnout. By prioritizing the backlog together, the team can make informed decisions about what work to focus on during the sprint, ensuring that the most critical features are delivered first.
References: Professional in Business Analysis Reference Materials source and documents.