Create a post method in Drupal 8
Solution:

We need a different path:

 * @RestResource(
 *   id = "custom_rest_resource_user_check",
 *   label = @Translation("Custom User Check"),
 *   uri_paths = {
 *     "https://www.drupal.org/link-relations/create" = "/api/v1/custom/user_check"
 *   }
 * )


Source:
https://www.drupal.org/node/2799383#comment-11619919