Creating ArPayment issue

[PeterMedhealth]

Hi,

I am trying to create an ArPayment object in .net c#.
We are generating 2 invoices - one for a positive value for the normal stand invoice | the other one negative value for credit to the previous invoice. Then, I'm trying to create ArPayment for the code however, In ArPaymentCreate ApplyToTransactions.ArPaymentItem has only one property for the stand invoice.

I am hoping to pass 2 invoices to linking together: let's say this payment for normal stand invoice but the fund the from credit invoice.
In API, ARPYMTDETAIL.[INLINEKEY] element is the one I wish to use in ArPaymentCreate. But ArPaymentItem has only one property.

• ApplyToRecordId: for invoice which payment applied

Can I use/extend/ custom fields?