You've got a fixed width and height set (1920x1080 like a full HD Screen!) so its overflowing.
Just change the width to "100%" and the height to something suitable and you'll be fine. it does seem to work in a markdown block as you suggest, or in an embed block and click the </> and paste your iframe code in there. not sure what plan you need to do that.
e.g.
<iframe width="100%"
height="500"
src="https://xd.adobe.com/embed/46fe53f9-9d26-4448-827f-88b5d8d04224-13ad/?fullscreen"
frameborder="0"
allowfullscreen>
</iframe>
Tuan is right though, it's definitely easier to help you if you do post a url and a temporary password, but I tried this and it works.
