Youtube, Vimeo and Animoto videos can all be embedded into your campaign, on the donation thank you screen. All you need to do is get the embed code for the video you would like to display. Generally, you can find this by clicking on the share button for the video.
Once you have the embed code, go to your campaign manager and select Edit Campaign Details
Scroll down to the Thank You Page Video box, then paste the embed code in. Update the campaign.