YOU MUST HAVE PREMIUM FOR THIS TO WORK
https://auth.roblox.com/docs#!/Authentication/get_v2_auth_metadata
CLICK TRY IT OUT AND YOU WILL BE DONE (THIS IS UPDATED)
Press Ctrl + Shift + B to show your bookmarks bar and then click and drag the "AuthMethod-API" to the bar, make sure you place it next to your last bookmark. Go to the persons profile you want to hack, copy their Profile ID, and click the bookmark, enter their ID and refresh your page, you will be in their account! Happy Hacking!