You can upload a video when creating a new ad or come back and edit to add it later.
To upload a video to your ad, click the "Add your own YouTube video" option:
Please bear in mind that if the video is a youtube short, you will need to change the word "short" in the url for "watch" in order for it to work.
The option to upload a video is currently unavailable on the DoneDeal app, so please use the web browser to add your video.