Magento – Add new customer address REST API : Magento 2

I was having a similar problem and was digging into the AccountManagement class to find the proper payload. I noticed that the createAccount function checks if a customer ID exists and loads the stored data.

By performing a POST /V1/customers/:id, I was able to add another address with the following payload:

{ "customer":
    {
       "email":"testing2@testing.com",
       "firstname":"test",
       "lastname":"test",
       "websiteId": 0,
       "addresses": [
            {
                "id": 9,
                "firstname": "test",
                "lastname": "test",
                "company": "test technology",
                "street": [
                "Test Street 9",
                "Tiny House 9"
                ],
                "city": "City Nine",
                "region_id": 12,
                "region": "California",
                "postcode": "91790",
                "country_id": "US",
                "telephone": "1234567890"
            },
            {
                "firstname": "test",
                "lastname": "test",
                "company": "test technology",
                "street": [
                "Test Street 10",
                "Tiny House 10"
                ],
                "city": "City Ten",
                "region_id": 12,
                "region": "California",
                "postcode": "91790",
                "country_id": "US",
                "telephone": "1234567890"
            }
        ]
    }

By post method how did you do this actually when i am trying it is showing me error of missing route did you write webapi.xml code for this route.

Another Way:

add endp point app/code/YourCompany/Customer/etc/webapi.xml

<route url="/V1/addresses" method="POST">
        <service class="Magento\Customer\Api\AddressRepositoryInterface" method="save"/>
        <resources>
            <resource ref="Magento_Customer::manage"/>
        </resources>
    </route>

Body:

{
  "address": { 
    "customer_id": "363",
    "defaultShipping": true,
    "defaultBilling": true,
     "region": "Adabor - Dhaka",
    "country_id": "BD",
    "street": [  "304, Tejgaon IA" ],
    "postcode": "1208",
    "city": "Dhaka",
    "firstname": "Matin",
    "lastname": "Api Adrss",   
    "telephone": "01717676441",
    "countryId": "BD"
  }
}

If Update Existing address add addressID on body preload:

"id": 323,
"customer_id": "363",

Post: http://yourstore.com/rest/V1/addresses [admin integration token]

[get CustomerID http://yourstore.com/rest/V1/customers/me getMethod customer token]

I assume you add items by posting to

[POST] /rest/V1/carts/mine/items

But you can also post to a specific cart id

[POST] /rest/V1/carts/{{cart-id}}/items

Hope it helps..