Backend-Api/ServiceHost/Hubs/HolidayApiHub.cs

using Microsoft.AspNetCore.SignalR;

namespace ServiceHost.Hubs
{
    public class HolidayApiHub : Hub
    {

        public async Task send(long id)
        {
            await Groups.AddToGroupAsync(Context.ConnectionId, GetGroupName(id));
        }

        public static string GetGroupName(long id)
        {
            return $"group-holiday-{id}";
        }
    }
}