An example of using Ask in Akka.Net with Web API

Endpoint.cs

public async Task<List<string>> RegisteredUsers(string serviceId)
{
  var system = Global.System;
  var serviceActorAddress = String.Format("/user/service/{0}", serviceId);
  var resp = await system.ActorSelection(serviceActorAddress).Ask(RegisteredUsersRequest.Instance);
  return resp;
}

ServiceActor.cs

public class ServiceActor : UntypedActor
{
  List<string> _registeredUsers = new List<string>();
  
  protected override void OnReceive(object message)
  {
    if(message is RegisteredUsersRequest)
    {
      Sender.Tell(_registeredUsers);
    }
  }
}