Squarespace allows you to add externally-hosted video to your website through its video content block. To add a video to your site, click the grey dot that appears in edit mode to add a new content block. From the options, choose video. Next, add your video's URL from the host site into the url section. You can choose to add a thumbnail image as a cover for the video block if you'd like, or you can leave it as is.
You can adjust the sizing of the video block using spacers.