Custom Button in CustomSiteConfig

<%- if @topic_view.topic.tags.present? %>
<%= t 'js.tagging.tags' %>: <%- @topic_view.topic.tags.each do |t| %> <%= t %> <%- end %>
<% end %>

Silverstripe Version:

Question:

Hey guys,
anyone some ideas or startingpoints for me for adding a custom button or action in a custom siteconfig extension? If it is possible it would be great if I could add that button after some fields in the fieldlist area. I need this for sending a request to a newsletter service for getting an auth. token.

Every idea, link or sources to read and learn would be Great!! Thank you!!
Here is what I already tried:

LeftAndMain:
    extensions:
        - CleverReachExtension

In my config.yml

... 
FormAction::create('loginCleverReach', 'Login'),
...

In my CustomSiteConfig updateCMSFields function. And this is my extension class:

<?php

class CleverReachExtension extends LeftAndMainExtension {

    private static $allowed_actions = array(
        'loginCleverReach'
    );

    public function loginCleverReach()
    {
        error_log('Test');
    }

}