Email forwarding using mailbox?

Is there a way to forward inbound emails to another email address using the mailbox ( not the control panel? I have a client who wants to be able to set up email forwarding for his emails. I checked out the email filters in mailbox but I was not able to just add a forward email, I have to enter something in the filter field.

You can set that filter so Subject Does Not Contain axiasdfnasdfnkdf or some other such nonsense. That should send all mail to the forward address.