Hi Linglish, Good luck with your exam.
regarding your question, the first input to stakeholder management is the well known commonly used input "Project management plan" which by definition includes all other subsidiary plans including the communication management plan. hope this clears your confusion.
reference: check section 13.2.1.1 from the latest PMBOK guide version. It describes why the Project management plan is an input to the stakeholder management, one of them is "Need and techniques for communication among stakeholders".