You don't have to do anything. You're not a resident of Loudoun County and your vehicle is registered in a county that does not require town or county stickers.
However, here is the page detailing what you would do if they actually wrote you a ticket:
https://www.loudoun.gov/index.aspx?NID=1929
If you are going to be there often or for more than a day or two, you should call the # (and email them) using the information in the "Visitors" section of the above link. That should prevent any actual tickets from being issued. If you do get one anyway they can resolve it.