We have define our API end point in etc/webapi.xml file. For Add product we have /V1/productalertstock/add/:productId and for delete /V1/productalertstock/delete/:productId. The complete URL will be http://{Magento2Root}/rest/V1/productalertstock/add/123

For Accessing above API You need Customer token, which can be retrieved by http://{Magento2Root}/rest/V1/integration/customer/token

Composer Install:

composer require pawan/module-product-alert-api
php bin/magento setup:upgrade
php bin/magento cache:flush

Where:

123 is the product ID
{Magento2Root} is root folder/Url of magento 2