The [tt_video_box video_url=""] shortcode has a base64 encode function that transforms the video iframe passed from this field.
Use the "Backend mode" and Video box to include an iframe...
Then switch to the "classic mode" to search and copy and use the generated base64 code.
Note: The best method is adding it using the Visual Composer's Video Box module, then switch to "Classic Mode" of the page editor and copy it as text if needed.