A Roblox Lua script running on your client has no direct Lua API or function to fetch another player's IP address. This is a deliberate and fundamental security boundary. So, how would a "real" IP grabber work? It would typically require a "man-in-the-middle" attack or tricking the user into clicking an external link.
It sounds like you’re referencing a related to tricking someone into clicking a link that appears to be a Roblox “IP logger” — but is actually fake (a “troll” version). fake ip logger troll script fe showcase roblox link