In.net Framework 4.5 using RestSharp version 105.2.3 getting System.MissingMethodException when I try to use request.AddJsonBody() method

I am using RestSharp version 105.2.3 in .Net MVC framework 4.5 project to integrate external API, when I use request.AddJsonBody(obj) I get the runtime exception as System.MissingMethodException

Following are the code I used to call API

```
public string CheckCMSUser(RequestObjectModel requestObject)
{
            
            RestClient client = new RestClient(requestObject.BaseURL + "CheckCMSUser");
            RestRequest request = new RestRequest();

            var payload = new RequestObjectCMSModel()
            {
                CompanyNumber = requestObject.CompanyNumber,
                UserAccountEmail = requestObject.UserAccountEmail,
                IncludeAllSubs = requestObject.IncludeAllSubs
            };

            
            request.RequestFormat = DataFormat.Json;            
            request.AddJsonBody(payload);
            
            request.AddHeader("WebAPI-Version", requestObject.WebAPIVersion);
            request.AddHeader("Authorization", requestObject.Token);

            var response = client.get(request);

            return response.Content;
}


public void CheckCMSUser()
{
            requestObj.CompanyNumber = "9999";
            requestObj.UserAccountEmail = "user@example.com";
            requestObj.IncludeAllSubs = "N";
            requestObj.Token = _workContext.Token;           

            ValidUserResponseModel response = JsonConvert.DeserializeObject<ValidUserResponseModel>(_externalApiService.CheckCMSUser(requestObj));
}
``` 

Please help me to solve this

Thanks in Advance