To enable forward posting you must first choose a transaction type to enable forward posting on. Once decided follow these steps:
From the Toolbar navigate to System Admin
Then to Setup
Then select Transaction Types from the list
Find the transaction type you want to enable forward posting on
On the right hand side of this screen there is an option Allow Advanced Postings
To enable Forward posting, set this to YES
This will allow transactions of that type to post into Periods forward in time, prior to running period ends.