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