ENDPOINT :  /rest/V1/guest-carts/{cart_id}/items

   e.g :    http://localhost.magento.com/rest/V1/guest-carts/DnsT4UjkMPPdID4K53h4ToaXMSElc8pI/items

METHOD : POST

HEADERS : Content-Type application/json

                    Authorization : Bearer 

PARAMETERS : 
 SKU
 QTY
 Quote_ID

PAYLOAD:
  {
  "cartItem": {
    "sku": "24-MB01",
    "qty": 1,
    "quote_id": "DnsT4UjkMPPdID4K53h4ToaXMSElc8pI"
  }
 }

Response :

{
    "item_id": 25,
    "sku": "24-MB01",
    "qty": 1,
    "name": "Joust Duffle Bag",
    "price": 80,
    "product_type": "simple",
    "quote_id": "12"
}  

                                           // return the item_id which will use to to update/delete item of the cart.