Sign up usually has one step where you enter your desired username and password, and a second step where the user receives a mail containing an url to click to confirm the given username and password. How exactly you implement your solution is up to you and your customer.
Check out the Marketplace module ‘Forget Password’ that will do this for you. or at least give you an example to copy from:
https://marketplace.mendix.com/link/component/1296/Mendix/Forgot-Password-module