You can't, but in any case there would be no need to, your ISP can't see anything past your firewall (or shouldn't be able to if you have it configured), and it's highly unlikely that they are going to start port scanning you in which case that would only show you had port 21 open regardless.
Its like asking the police to look inside the shop with its doors closed as they drive past, sure they can see who's coming in an out, what they are wearing or carrying but once inside what goes on is unbeknownst to them, unless someone tells them.
If they wanted to they could simply inspect the source/target packet data and type being passed from the server to your router, that is more likely to be the case.