Magento 2 REST API – How to Get Store ID

Magento 2 allows some web APIs to be accessed by unauthenticated (anonymous) users. Many of these APIs allow a customer to have a robust shopping experience on the website without having to log in.

Please check this link https://devdocs.magento.com/guides/v2.3/rest/anonymous-api-security.html

Preventing anonymous access to these endpoints could cause third-party integrations to fail. If a third-party integration calls any of these endpoints, it will receive an authentication error instead of the expected response. In this case, you might need to disable this feature.

To disable this feature, log in to the Admin panel and navigate to Stores > Settings > Configuration > Services > Magento Web API > Web API Security. Then select Yes from the Allow Anonymous Guest Access menu.

No need for any custom API, you can use the customer edit endpoint

EndPoint  = "/V1/customers/{customer_id}"

Method = "PUT"

Json  = 
{
  "customer": {
    "id": {customer_id},
     "email": "{customer_email}",
    "firstname": "{customer_firstname}",
    "lastname": "{customer_lastname}",
    "store_id": {store_id},
    "website_id": {website_id},
    "extension_attributes": {
     "is_subscribed": true
    }
  }
}

Please replace {customer_id}, {customer_email}, {customer_firstname}, {customer_lastname}, {store_id}, {website_id} with the customer information.

this was tested and worked for me