Thanks a lot bbozo 😀 .
You are right from the Interface, it directly goes well.
I didn’t know where to find the PREROUTING in Zeroshell.
Now, my FTP server is available from the outside.
Solution gave just before (abstract) :
On the left, section ‘Network‘, menu ‘Router‘
At the top, select the tabbed panel named ‘Virtual Server’
and enter the redirection.