curl --request POST \
  --url https://api.returnshelper.com/uat/user/api/ReturnInventory/assignReturnInventorySku \
  --header 'Content-Type: application/json' \
  --header 'x-rr-apikey: <api-key>' \
  --header 'x-rr-apitoken: <api-key>' \
  --data '
{
  "returnInventoryId": 123,
  "sku": "<string>"
}
'