{0} should get replaced with the DataValueField value (EventID in your case). See the following code snippets:
DayPilotScheduler.cs
string val = DataBinder.GetPropertyValue(dataItem, DataValueField, null);
// ...
((ArrayList)ViewState["Items"]).Add(new Event(val, start, end, name, resourceId));,
Event.cs
public Event(string pk, DateTime start, DateTime end, string name, string resource)
{
this.PK = pk;
this.Start = start;
this.End = end;
this.Name = name;
this.Resource = resource;
}
DayPilotScheduler.cs
output.AddAttribute("onclick", "javascript:event.cancelBubble=true;" + String.Format(EventClickJavaScript, p.PK));