I run this async function from web service project:
    [WebMethod]
    public static async Task<string> CreateEventsAsync(Event @event, string accessToken)
    {
        try
        {
            var graphClient = GetGraphServiceClient(accessToken);
            var NewEvent = await graphClient.Me.Calendar.Events
                 .Request()
                 .AddAsync(@event);
            return NewEvent.Id;
        }
        catch (Exception e)
        {
            throw;
        }
    }
and the project break and return this result:
<?xml version="1.0" encoding="UTF-8"?>
<TaskOfString xmlns="http://api.tipulog.mkhltd.co.il/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"/>
