When I setup email forwarding the automatic address that Freshdesk sets up changes.

As soon as you sign up, you would have your default support email address as support@domain.freshdesk.com

But as soon as you update it to a custom email address like support@domain.com, it gets jumbled to domaincomsupport@domain.freshdesk.com

The prefix before @ would act as the key identifier, in case if you are assigning tickets to groups at the time of ticket creation itself and the suffix after @ would tell you to which Freshdesk instance this should be sent to.

Therefore by adding the jumbled forwarding address in your forwarding rule (In the mailbox), when the customer sends an email to  support@domain.com, it automatically gets created as tickets in Freshdesk.