Call Web Services on post first publish

1. To add a checkbox you need to use the add_meta_box function, with this you can create a metabox for the new post screen. You have to create a function that generates the checkbox element, and that’s it.
2. You can use the action publish_post to call the services you need when it happens, you have other options, like save_post. It really depends on how flexible you want your code to be.
3. In step one you created a custom metabox, when you wrote that function, you should’ve created an input element with a name, so, you can access the value of the checkbox from the POST array, like this $_POST['name_of_the_field'].

4. If you use the publish_post action, you can get the Post data like this:

   add_action( 'publish_post', 'post_publish_func', 10, 2 );
   
   function post_publish_func($id, $post){
   
   $title = $post->post_title; //or get_the_title()
   $categories = get_the_category($id);
   $checkbox = $_POST['name_of_the_checkbox'];
   
   //magic goes here
   }
   

Be careful using get_the_title or $post->post_title, because if you are modifying a post, get_the_title gets the value already stored in the DB (the title before you made the modifications) and $post->post_title gets the value of the modified version. This happends with other methods too, get_the_content for example.