In burp click on forward until you see the post request. Hit Ctrl + i to send it to the Intruder tab
Goto Intruder, choose Cluster bomb as Attack type. Clear all variables and add the value of username and the value of password. For both we want to provide a wordlist.
gunzip /usr/share/wordlist/rockyou.txt.gz
head -20 rockyou.txt >passwords.txt