I'm not sure how to bring up a chat with window if you don't have a search result from that person. Saving the ISP (I assume you mean IP, like 12.34.56.78) should help but I don't know where to enter it. You could try connecting to the IP in the Connections panel, searching for something that would match, and right-click a search result for Chat With Host but this is ridiculously complicated for something that should be so simple. It seems like a chat with IP feature is missing.