Customer custom attribute value did not save using rest api in magento 2.3?

Customer custom attribute value did not save using rest API in Magento 2.3?

API URL: http://127.0.0.1/magento2/rest/V1/customers

Method: POST

My POST JSON request data is :

{
    "customer": {
        "email": "[email protected]",
        "firstname": "Tedst2",
        "lastname": "yosur",
        "store_id": 1
    },
    "password": "admin@123",
    "custom_attributes": [{
            "attribute_code": "test_attribute",
            "value": "simple"
        }
    ]
}

Setup-Script

public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {

        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

        $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();

        /** @var $attributeSet AttributeSet */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);

        $customerSetup->addAttribute(Customer::ENTITY, 'test_attribute', [
            'type' => 'varchar',
            'label' => 'test attribute',
            'input' => 'text',
            'required' => false,
            'visible' => true,
            'user_defined' => true,
            'sort_order' => 1000,
            'position' => 1000,
            'system' => 0,
        ]);
        //add attribute to attribute set
        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'test_attribute')
        ->addData([
            'attribute_set_id' => $attributeSetId,
            'attribute_group_id' => $attributeGroupId,
            'used_in_forms' => ['adminhtml_customer'],
        ]);

        $attribute->save();


    }

The Customer is created but the test_attribute is not "simple". Is there anyone who had the same problem and fixed it? Please let me know how to resolve it?.or any another way to save?

{ 
  "customer": 
  {
  "email": "[email protected]", 
  "firstname": "x", 
  "lastname": "y", 
  "website_id":1, 
  "group_id":1, 
  "custom_attributes": 
  [ 
    { 
    "attribute_code": "test_attribute", 
    "value": "simple" 
    } 
  ]

},
"password": "admin@123"
}