If you are trying to permit a specific website to connect to your computer via a particular port number, I assume your firewall wants to know the !P (or 'source') address of that site in order to 'recognise' it when it attempts to connect.
Most I P addresses currently consist of four groups of numbers - for example, Google.co.uk's address is 209.85.229.104 If the site you want to allow to connect doesn't provide an IP address, there are various free tools which can locate them, and a Google search under 'IP Addresses' should produce some.
One I have used is ID Serve, which I got from Steve Gibson's website;
http://www.grc.com/id/idserve.htm