How to retrieve the content (with a specific ID) via ajax by clicking a link tag

I would put the action in the post data

$.ajax({
  url: "/wp-admin/admin-ajax.php",
  type:"POST",
       data: { 
       action: "my_custom_data",
       post_link: post_ID
},
success: function (response) {
        console.log(response);
        $('#post-data').append(response);
    }
});
   return false;
.....

Then use $_POST[‘post_link’] in your PHP

   function my_custom_data(){
       $post_link = $_POST['post_link'];
   echo get_the_content($post_link);
   die();
   }