How to create a rule for FTP

More information would be helpful. (And I hope you're not using FTP, which is very insecure, but rather SFTP, right?)

Assuming you're using IPv4, you'll need to set up a NAT rule to map from the public (ISP) port on your firewall to the port on your internal server. And then a firewall rule to allow that traffic to happen. The NAT Rule page has a helper when you pull down the "Add Rule" button that will help you create both if you check the right boxes.

I have created the rule in firewall. It is accessible but not able to access in Web browser

You need DNAT and firewall rule, can you show us your definition(s)?

Do you have a public DNS entry for your server?

Dns entry is already put in the server but still the same problem

OK, can you post your DNAT rule and the firewall rule, then?

Try to obfuscate the IPs like 204.xx.yy.23 and 192.168.aa.17, then we can easily recognize which IP is public and which is private.