Change your client code like this.
var _stDeveloperApi = new RestClient("http://127.0.0.1/");
var url = string.Format("api/v1/SignalR/MultiClickMode");
var listOfSelectedTicketsIds = new List<int> { 2, 3 };
var request = new RestRequest(url, Method.GET);
listOfSelectedTicketsIds.ForEach(t =>
request.AddParameter(
"listOfSelectedTicketsIds", t, ParameterType.GetOrPost));
var response = _stDeveloperApi.Execute(request);
Change the web API action method signature like this.
public HttpResponseMessage MultiClickMode(
[FromUri]List<int> listOfSelectedTicketsIds)