If you are starting a meeting as an API user, you have already authenticated through the REST API when retrieving your ZAK token. For this reason, you are not required to include the password, since you are already authorized to start the meeting on behalf of the host’s account.
If you are required to provide a password (e.g. you are a participant joining someone else’s meeting), you would need to use an instance of JoinMeetingParams and the joinMeetingWithParams method.