There are several options:
Before you forward the message, set the reply-to email address to the email from the client and create a filter in osTicket to look for the reply-to address.
Another option is to share the mailbox from your ticket system, so you can copy/move the email from your own inbox to the ticket inbox. That is not possible in every environment, but when its possible, I think it's the (depending on the mail client) easier then setting the reply-to address.