HiPay Backoffice
In order to capture an authorized transaction you should go to the Transaction Preview.
Here, you will have a capture button, click on it to do a manual capture of the funds after a service has been rendered or a product has been sold.
API Request
In order to capture an authorized transaction you will need to use the Maintenance API.
Full capture: you can only send the parameter operation set to capture.
Partial capture: you should send the parameter operation set to capture, the amount you want to capture and the currency.