To get In Meeting Chat Messages via the Zoom API, ensure the meeting is recorded, and the "Save chat messages from the meeting / webinar" setting is enabled.
After the meeting is over and the recording files are finished processing, call the GET Meetings Recordings endpoint. You can also use the Recording Completed webhook. In the payload you will see the file_type of "CHAT" which you can then download via the download_url.
If you are looking to get Zoom Chat, chat messages, you can use the Chat APIs.