We've got a form you can fill out for just such an occasion: https://support.deadbydaylight.com/hc/en-us/requests/new?ticket_form_id=191426

You'd still need to report them in-game, of course, but this is the best way to attach a video to your report. This way you can keep playing with the video uploads rather than waiting to paste it into the report box.

There is a file size limit on attachments. I would recommend uploading it to a site like YouTube (you can set the video to unlisted if you'd like) and then posting the link in the ticket.