Don’t do this.
There’s no reason to have players sign up via a Roblox game.
If you want accounts for ingame purposes, use a combination of user id pulled from the player instance (this cannot be spoofed, so it is secure), and some kind of unique key (ie. custom profile name, PIN, save slot).
If you’re making accounts for a website, have users sign up via the website (ideally through another service such as Google or Twitter so you don’t have to handle passwords).
Asking for a password in a Roblox game is a fast track towards getting reported for scamming, even if that’s not your intention. Users will use their Roblox password in your game. It’s inevitable.