Create products through API with xml payload

Question

We are trying to create products by API, if we use json as payload, no problem Endpoint:

POST /rest/default/V1/products

Payload:

{
  "product": {
    "sku": "MS-Champ-S",
    "name": "Champ Tee Small",
    "attribute_set_id": 9,
    "price": 25,
    "status": 1,
    "visibility": 1,
    "type_id": "simple",
    "weight": "0.5",
    "extension_attributes": {
        "category_links": [
            {
                "position": 0,
                "category_id": "11"
            },
            {
                "position": 1,
                "category_id": "12"
            },
            {
                "position": 2,
                "category_id": "16"
            }
        ],
        "stock_item": {
            "qty": "10",
            "is_in_stock": true
        }
    },
    "custom_attributes": [
        {
            "attribute_code": "description",
            "value": "The Champ Tee keeps you cool and dry while you do your thing. Let everyone know who you are by adding your name on the back for only $10."
        },
        {
            "attribute_code": "tax_class_id",
            "value": "2"
        },
        {
            "attribute_code": "material",
            "value": "148"
        },
        {
            "attribute_code": "pattern",
            "value": "196"
        },
        {
            "attribute_code": "color",
            "value": "52"
        },
        {
            "attribute_code": "size",
            "value": "168"
        }
    ]
  }
}

But we use an XML as payload, we face an error.

<?xml version="1.0" encoding="UTF-8"?>
<magento_api>
   <product>
      <attribute_set_id>9</attribute_set_id>
      <custom_attributes>
         <element>
            <attribute_code>description</attribute_code>
            <value>The Champ Tee keeps you cool and dry while you do your thing. Let everyone know who you are by adding your name on the back for only $10.</value>
         </element>
         <element>
            <attribute_code>tax_class_id</attribute_code>
            <value>2</value>
         </element>
         <element>
            <attribute_code>material</attribute_code>
            <value>148</value>
         </element>
         <element>
            <attribute_code>pattern</attribute_code>
            <value>196</value>
         </element>
         <element>
            <attribute_code>color</attribute_code>
            <value>52</value>
         </element>
         <element>
            <attribute_code>size</attribute_code>
            <value>168</value>
         </element>
      </custom_attributes>
      <extension_attributes>
         <category_links>
            <element>
               <category_id>11</category_id>
               <position>0</position>
            </element>
            <element>
               <category_id>12</category_id>
               <position>1</position>
            </element>
            <element>
               <category_id>16</category_id>
               <position>2</position>
            </element>
         </category_links>
         <stock_item>
            <is_in_stock>true</is_in_stock>
            <qty>10</qty>
         </stock_item>
      </extension_attributes>
      <name>Champ Tee Small</name>
      <price>25</price>
      <sku>MS-Champ-S</sku>
      <status>1</status>
      <type_id>simple</type_id>
      <visibility>1</visibility>
      <weight>0.5</weight>
   </product>
</magento_api>

error message:

An empty custom attribute is specified. Enter the attribute and try again.

we are able to create customers the both, without any problem