Thanks. But it doesn't work.

The operation cannot be completed:\n- Create/update: a mandatory field is not set.\n- Delete: another model requires the record being deleted. If possible, archive it instead.\n\nModel: Activity (mail.activity)\nField: Document Model (res_model_id)\n

I tried to add the missing field but that just brings me back the display_name error.
Even copilot and chatGpt recomment mine or your solution but it doesn't work.
I wonder if this is a bug in the Odoo api.

Try this

{

  "jsonrpc": "2.0",

  "method": "call",

  "params": {

    "service": "object",

    "method": "execute_kw",

    "args": [

      "<dbname>",

      2,

      "<apikey>",

      "mail.activity",

      "create",

      [

        {

          "res_id": <contactid>,           

          "res_model": "res.partner", 

          "activity_type_id": 2, 

          "summary": "<text>",

          "note": "<text>"

        }

      ]

    ]

  },

  "id": 1

}