In Service Rest - under method request tab get the incoming request in parameter and in service activity first step place the property-set to convert the JSON to page/pagelist. Attached scree-shot for reference.
@BoukhroufH you would have called an activity in the respective method (Post for example) in Service Rest. so you can call the data transform in that activity and do the required mapping. you need to pass two parameters to the data transform. Refer the attachment.