Hello Neil German,
This can be achieved by doing small setup in Terms of payment form in AP.
Before that; need to know the following;
--- What is the terms of payment you have set for the vendor for which you are posting the PO invoice?
--- Is there any specific "Main account" your customer is preferred where payment journal will be recorded automatically?
In "Terms of payment" form; you have to enable the Cash payment check box for payment method "COD". Then if there is any specific account is available to record the payment; that you have to select in the Cash field from the drop down menu.

Once this is done; link this terms of payment to the vendor master.
Then simply post the PO invoice with that vendor.
When the invoice is posted; just generate the account statement for the account which you mapped in the terms of payment "COD". You will see the payment journal is automatically posted in that account for that invoice.
Below is the test result from my system.

The above vendor balance is BHD 1575.000
Now when I generate the account statement for account "1102102", it shows the posted payment voucher for this invoice.

You can also check this voucher in General ledger > Inquiries and reports > Voucher transactions.

Kindly check in your environment and provide your feedback.
Best regards,
Sourav Dam
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