Allocation - How to include Open Item Key

Hello,

I have a question for the experts regarding Allocations. On the «Offset» page I set the Account and Business Unit values as required. The account however is an Open Item account and therefore I need to set the «Open Item Key» value. But the field does not show up in the dropdown so I can't set the value. Any ideas So far the best I can do is to put the Vendor ID value in the field «Journal Line Reference» but this does not help me much. Here is additional information about the type of allocation:

• Pool Allocation Type=Copy