That is a default vBulletin feature - you will just need to make some edits to the postbit template.
It is called User Profile Fields.
AdminCp -> User Profile Fields -> Add new User Profile Field -> Select the type you want and click 'Continue' -> Edit the form to your wishes and click Save.
And then follow this article:
https://vborg.vbsupport.ru/showthread.php?t=118896
If you need any helps on any of the steps, just reply here.