Step1. Enter ftp.addondomain.com in the Hostname field. (Eg: ftp.romeospanel.com)
Step2. Addondomain(without tld)@primarydomain.com (Eg: romeospanel@hostromeo.com)
Step3. Enter Password in the password field.
Step4. Then click save and check if the values entered in that field looks like this: (Addondomain (without tld)@primarydomain.com/addondomain(without tld))
(Eg: romeospanel@hostromeo.com/romeospanel)