Open source code for plugin, find where calls to obs_websocket_vendor_register_request() are happening https://github.com/Aitum/obs-aitum-stream-suite/blob/3435adaa9a7b5c1f040af32fd07c195f0ba82ef7/src/utils/obs-websocket.cpp#L835

in this case we are looking to add chapter markers to aitum suite recordings. in this example search the repo for vendor_request_add_chapter uses. You will find this https://github.com/Aitum/obs-aitum-stream-suite/blob/3435adaa9a7b5c1f040af32fd07c195f0ba82ef7/src/utils/obs-websocket.cpp#L349

without getting into it, you can see 2 strings, "output" and "chapter_name" these are the required to be passed in by the websocket command

You can send the command with a obs websocket lib or use streamer bot.

{
  "requestType": "CallVendorRequest",
  "requestData": {
    "vendorName": "aitum-stream-suite",
    "requestType": "add_chapter",
    "requestData": {
      "chapter_name": "TEST ",
      "output": "-recording"
    }
  }
}

the vendor name was found in first file above https://github.com/Aitum/obs-aitum-stream-suite/blob/3435adaa9a7b5c1f040af32fd07c195f0ba82ef7/src/utils/obs-websocket.cpp#L835. search for obs_websocket_register_vendor in the repo.

"add_chapter" was also from the file above https://github.com/Aitum/obs-aitum-stream-suite/blob/3435adaa9a7b5c1f040af32fd07c195f0ba82ef7/src/utils/obs-websocket.cpp#L835