I have a a view model that looks like.
public class StoreItemViewModel
{
    public Guid ItemId { get; set; }
    public List StoreIds { get; set; }
         
         
I managed to get this working. I don't think its the correct way but it works.
 public static IRestResponse Create(object objectToUpdate, string apiEndPoint) where T : new()
    {
        var client = new RestClient(CreateBaseUrl(null))
        {
            Authenticator = new HttpBasicAuthenticator("user", "Password1")
        };
        var json = JsonConvert.SerializeObject(objectToUpdate);
        var request = new RestRequest(apiEndPoint, Method.POST);
        request.AddParameter("text/json", json, ParameterType.RequestBody);
        var response = client.Execute(request);
        return response;
    }