To post return orders into Microsoft Business Central the URL POST http://<serer name>:<odata port>/<service instance name>/api/BEterna/Import/v2.0/companies(<company system id>)/returnOrders can be used. 
It must be called with a JSON structure to create an order (webshopChannelCode is mandatory):
{
   "webshopChannelCode": "B2B",
   "subclient": "",
   "customerIP": "",
   "language": "DEU",
   "testOrder": "0",
   "orderId": "D-0000002",
   "orderType": "2",
   "orderDate": "2022-07-07 13:10:00",
   "currency": "EUR",
   "customerComment": "",
   "customerNo": "C00010",
   "registeredCustomer": "1",
   "contactNo": "CT000023",
   "returnOrderLines" : [
      {
         "id" : "10000",
         "type" : "1",
         "itemNumber" : "1900-S",
         "variant" : "",
         "amount" : "2",
         "orderNo": "101008",
         "orderLineNo": "10000",
         "returnReason": "Storno",
         "lineDescription" : "Desc1",
         "lineDescription2" : "Desc2"
      }
   ]
}
If a field is empty it can be left out in the JSON structure. The Return Header fields have the following meaning:
| field | data type | content | 
|---|---|---|
| webshopChannelCode | Text[30] | Webshop Channel Code | 
| subclient | Text[30] | the subclient who placed the order, actually not used | 
| customerIP | Text[30] | IP address of the customer, actually not used | 
| language | Text[3] | language code of the customer | 
| testOrder | Text[30] | 0or1, actually not used | 
| orderId | Text[30] | distinct ID of the order | 
| orderType | Text[30] | type of the order, type 2 is for return order | 
| orderDate | Text[30] | order date and time in the form 2022-05-01 10:00:00 | 
| currency | Text[30] | the ISO3 currency code of the order | 
| customerComment | Text[80] | special customer comment for the order | 
| customerNo | Text[20] | customer no. of the order, must be filled, even if the customer is new | 
| registeredCustomer | Text[30] | 1if the customer is registered and must exist,0if it is a guest order | 
| contactNo | Text[20] | contact no. of the order, must be filled and exist | 
There can be one or more Return Lines. They are transferred using a JSON array. The following fields can be defined for a single order line:
| field | data type | content | 
|---|---|---|
| id | Integer | distinct identifier of the line in the order, must be distinct per order | 
| type | Text[30] | must be one of the following values: 1item line,2voucher sale,3discount,4shipping costs,5voucher cashing in,6payment costs, which values are possible depends on the setup of the webshop channel | 
| itemNumber | Text[30] | the item number for item lines, must be empty otherwise | 
| variant | Text[30] | optionally the item variant code for item lines, must be empty otherwise | 
| amount | Text[30] | the quantity of the line for item lines, otherwise ist must always be 1 | 
| orderNo | Text[30] | the order no. connected to posted sales document | 
| orderLineNo | Text[30] | the order line no. in order document | 
| returnReason | Text[30] | reason of returning order line | 
| lineDescription | Text[250] | the first description of line | 
| lineDescription2 | Text[250] | the second description of line | 
The request is answered with a JSON structure containing all the header data that were transferred. If the answer should contain the transferred order lines as well, the $expand=returnOrderLines can be attached to the URL and the answer contains the full order data that were transferred:
{
    "@odata.context": "http://<server name>:<odata port>/<service tier name>/api/BEterna/Import/v2.0/$metadata#companies(<company system id>)/returnOrders/$entity",
    "@odata.etag": "W/\"JzE5OzczMTc3ODQ0MzEzODk0ODgxMTIxOzAwOyc=\"",
    "systemId": "a928cfd6-f7fd-ec11-be4d-a542e7ee7cff",
    "webshopChannelCode": "B2B",
    "subclient": "",
    "customerIP": "",
    "language": "DEU",
    "testOrder": "0",
    "orderId": "D-0000002",
    "orderType": "2",
    "orderDate": "2022-07-07 13:10:00",
    "currency": "EUR",
    "customerComment": "",
    "customerNo": "C00010",
    "registeredCustomer": "1",
    "contactNo": "CT000023",
    "returnOrderLines": [
        {
            "@odata.etag": "W/\"JzIwOzE1MjIyNDE3ODU0MjI0NzMwNTU0MTswMDsn\"",
            "systemId": "aa28cfd6-f7fd-ec11-be4d-a542e7ee7cff",
            "id": 10000,
            "type": "1",
            "itemNumber": "1900-S",
            "variant": "",
            "amount": "2",
            "orderNo": "101008",
            "orderLineNo": "10000",
            "returnReason": "Storno",
            "lineDescription": "Desc1",
            "lineDescription2": "Desc2"
        }
    ]
}