How to get attributes before adding product to the cart past api / rest?

[SergioMordente]

I've been researching and testing plugins and events for more than 20 days add the customized item passed by POST /V1/carts/{quoteId}/items

{
  "cartItem": {
    "sku": "1809",
    "qty": 1,
    "quote_id": "20897", "extension_attributes": {service_order ":" 33333333 "
    }
  }
}

extension_attributes only exists in the API / Rest call and must be saved in the quote_item table. Is there a way to get the POST data?