// setting the current context MUST be done in the constructor and is only valid for Synchronous events
        private HttpContext currentHttpContext = null;

        public EventReceiver1(): base()
        {
            currentHttpContext = HttpContext.Current;
        }

// this in your actual event function
string MyValue = currentHttpContext.Request.QueryString["Parameter"];

only works in async. events. Ending in "ing"

Getting Query String from Event Receiver

Development, sharepoint

C#