Addind script to Footer.php in child theme (Onclick Mailchimp Subscribe Popup)

Hi,

I am using Mailchimp and want to get the Mailchimp subscribe popup working on onclick
(the standard popups can only be triggered immediately, after 5 seconds, after 20 seconds, after the visitors scrolls to the middle of the page, but not on a click.)

To get this working a litte workaround is necessary, which is described here.

The workaround works for me, the only problem is that it is necessary to add this script to the footer.php:

<script type="text/javascript" src="//downloads.mailchimp.com/js/signup-forms/popup/embed.js" data-dojo-config="usePlainJson: true, isDebug: false"></script>
<script>
function showMailingPopUp() {
  require(
    ["mojo/signup-forms/Loader"],
    function(L) {
      L.start({"baseUrl":"mc.us16.list-manage.com","uuid":"YOUR_UUID","lid":"YOUR_LID"})
    }
  );
 
  document.cookie = 'MCPopupClosed=;path=/;expires=Thu, 01 Jan 1970 00:00:00 UTC;';
  document.cookie = 'MCPopupSubscribed=;path=/;expires=Thu, 01 Jan 1970 00:00:00 UTC;';
}
     
document.getElementById("open-popup").onclick = function() {showMailingPopUp()};
</script>

I would like to add this code to the footer.php of the child theme instead of the mother theme but I don’t know how to do that properly.

Basically I know how to add a file to my child theme, but I am not sure how exactly the content of the footer.php should look like.
Can I just copy the scipt in a empty footer.php file, or are there some other code lines necessary.

I implemented the button here on this site.
Login-data in the private content.

Looking forward for your suggestions.