How to write account rule based on PO Charge Account

Summary:

I have a requirement to default account on my AP prepayment to 123456 when PO charge account is 564321. And AP prepayment should default to 123457 for all other PO Charge account.

I created an account rule where I wrote source as Constant= 123456 for condition "PO Charge Account"."Natural Account Segment" = 564321. And second line for source as Constant=123457 with no condition.

I could not find source as PO Charge Account under Payables subledger so that I picked it from Purchasing subledger application while application for Account Rule still says Payables. As I used Purchasing subledger application in Source, I am not able to select this account rule Subledger Journal Entry Rule Set for Payables.