ASP.NET Core 3.1: Web Api: Same Post Method: Multiple types of Json Objects

I have a requirement where my POST method(single endpoint) needs to accept different types of json strings and act on them. I tried the following way and am getting a 404 error:

using Newtonsoft.Json.Linq;

public async Task<IActionResult> Post([FromBody] string jsonString)
    {
        if (string.IsNullOrEmpty(jsonString)) return BadRequest();
        try
        {
            JObject _jsonObject = JObject.Parse(jsonString);
            string _response = string.Empty;

            if(_jsonObject != null)
            {
                string messageType = _jsonObject.GetValue("objectType").ToString();
                if(messageType.ToLower() == "type1")
                {                        
                    _response = await dataRepository.InsertType1Record(_jsonObject );                        
                } else if (messageType.ToLower() == "type2")
                {
                    _response = await dataRepository.InsertType2Record(_jsonObject );                        
                } else if (messageType.ToLower() == "type3")
                {
                    _response = await dataRepository.InsertType3Record(_jsonObject );
                }
                return Ok(_response);
            }
            return BadRequest();
        }
        catch (Exception e)
        {
            _logger.LogTrace(e, "Exception Error");
            return BadRequest();
        }

    }

When I tested this out in Fiddler, I keep getting a 404 error. My request:

Host: localhost:44307
Content-Length: 1563
content-type: text/plain

{
    "objectType": "type1",
    "objectDetails": [
        {
         "field1": "value1",
         "field2": "value2"
        },
       {
         "field1": "value3",
         "field2": "value4"
        }
    ]
}
OR
{
    "objectType": "type2",
    "headerField1": "headerValue1",
    "headerField2": "headerValue2",
    "objectInfo": [
        {
         "field3": "value1",
         "field4": "value2",
         "field5": "value3"
        },
       {
         "field3": "value7",
         "field4": "value8",
         "field5": "value9"
        }
    ]
}

How do I accomplish this? Any help is appreciated.

Edit: Instead of using Newtonsoft.Json, I used the in-built JsonSerializer and did it the following way:

 public async Task<IActionResult> Post([FromBody] System.Text.Json.JsonElement jsonString)
    {            
        try
        { 
            string _jsonString = System.Text.Json.JsonSerializer.Serialize(jsonString);
            string messageType = jsonString.GetProperty("objectType").GetString();

          ......
       }