you can write your custom string into the field Type. When the ICC paragraph will be rendering the list of your items there will be request sent to the ERP with your custom string passed into it.
So the ERP will receive the request like that:
<GetList type="yourType" ...></GetList>
And you need to make the ERP to respond in the same format as it does for the supported types like OpenOrder, Credit, Invoice.
You can read more about the Integration customer center module here: https://doc.dynamicweb.com/documentation-9/integration/integration-framework-1/integration-customer-center