can we have a page that..
the user even have not login yet, but they can fill in they user name, contact number/ email, then complaint, with one click, it can direct send to the predefined email address?
Are you making a feature request to Mikrotik, or are you asking how to solve the problem yourself?
If the latter: you can’t do that on the router itself. Servlets hosted by the Hotspot cannot access CLI tools to send email, and there are no email functions within the Hotspot. You’d have to host this page somewhere else, on some server of yours. Simply whitelist that server in the walled garden. There are dozens of canned solutions for any language you’d want (Python, PHP, Perl, Ruby, whatever) out there.