const options = {
  method: 'POST',
  headers: {'x-spree-api-key': '<api-key>', Authorization: 'Bearer <token>'}
};

fetch('http://{defaultHost}/api/v3/admin/product_types/{id}/apply_to_products', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request POST \
  --url http://{defaultHost}/api/v3/admin/product_types/{id}/apply_to_products \
  --header 'Authorization: Bearer <token>' \
  --header 'x-spree-api-key: <api-key>'

{
  "products_count": 0
}