How to set the payment method on an invoice created in Dexxter

Hello, I would like to know how I can set a payment method when entering a private person’s invoice that I create myself in Dexxter. And how can I keep a cash book in Dexxter that is linked to it? If it is done by bank transfer, I can add a screenshot from the bank to the invoice, but I cannot do that with cash.

Many thanks in advance.