JS: Can't prevent form from being submitted

I am in the process of rewriting one of my addons, as jquery is no longer used. A modal should be displayed as soon as attachments are uploaded to a post. Sending the form with the message should be prevented until the modal dialog is confirmed with "ok".

And this is exactly where my problem lies. I can no longer prevent the form from being sent.

I have looked at this area (row 145) in the Attachment Manager (/js/xf/attachment_manager.js):
JavaScript:

XF.on(this.form...

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