Currently this isn't possible. The biggest reason is security. Their embed code uses a script, and arbitrary script code is not something we will allow until we find a way to properly sandbox it. We've been researching ways to allow custom scripts, isolated in their own iframe sandbox, but I don't have a timeline for when something like that might be available.
The workaround for now is to host a static site somewhere (S3 is a very cheap option) that includes this embed code. You can then using the External Website block to display that URL.