CoreEvents inherit VIEW_INJECT_CUSTOM_CONTENT for creating a filter option in Campaigns

I need to create a filter in Campaigns page using mautic CoreEvent, VIEW_INJECT_CUSTOM_CONTENT / VIEW_INJECT_CUSTOM_BUTTONS. With that note, need to design a dropdown button. There is no available samples or plugins which overrides existing functionality.
I created a EventListener with name
'class CustomContentSubscriber extends CommonSubscriber'
where the event is called from getSubscribedEvents() calling a function

CoreEvents::VIEW_INJECT_CUSTOM_CONTENT => [‘getContentInjection’, 0],

public function getContentInjection(CustomContentEvent $event)
switch ($event->getViewName()) {
case ‘MauticCampaignBundle:Campaign:list.html.php’:
switch ($event->getContext()) {
View page:

Dropdown button

This doesnt populate the dropdown in the required place. Near contacts source filter in campaigns page. Can someone help in this aspect?