To add a video to your form, head over to your Google Drive account and click on 'create' then select forms. Select a template for your form and move on to next page where you have to click on "add item" and choose video
Now you can either search for YouTube videos right from the popped-up pane or copy and paste the URL of the video you want to use.
This is how the video will look like inside your form
You can also change the placement of the video by moving it to the left, right or centre