You need to do this on the Seller Hub here:
https://www.ebay.co.uk/sh/ovw
You may need to add a pjoto first and then the option to add a video should be available.
Use a PC/ laptop too not the app or mobile browser, Chrome works best with eBay.
There's also some more information the link below.
You can only add one video per listing, but you can use the same video on multiple listings - you just need to upload it once. Upload a video using the new listing tool by selecting Add video from the Photos &
Video section: - The video should be .mp4 or .mov with a 150MB limit, or about one minute long - https://www.ebay.com/help/selling/listings/creating-managing-listings/add-video-to-listing?id=5272
@bizybee2014