The "Source Code" Feature on your app editor will allow you to paste in raw HTML code.
This is especially useful for embedding things like Livestream video players or other sources of media hosted externally.
Here's a helpful video overview that covers the basics of the "Source Code" feature:
If you're used to working with HTML code, this is your playground. Have fun with it!
If not, don't worry - there are still plenty of other tools elsewhere in the editor that will help you customize your app. Head back over to the Features Overview page to learn more.