You may also select an account on the Batch entry screen (right-click on selected transaction) and select the "Select from account open item list" option. The "Open item selection" screen will list the unlinked transactions as well as transactions with outstanding amounts.  

Select the transaction(s) on the "Open item selection" screen.

Add a tick in the "X" column to link.

Click OK to link the transaction in the Batch to the selected transaction(s) in the "Open item selection" screen.