Chatsongmusic can you describe your use case in more details?
The Flarum SPA is not designed to be run entirely from an iframe. There will likely be some cross origin, cookie access and browser permissions issues, as well as inability to use all of the responsive or touch features of Flarum and its extension. Deep linking to anything will also prove impossible.
If you only want to embed a specific discussion into a page as a comment thread, the
flarum/embed extension provides such a feature, though it's not really documented anywhere and should probably be more considered an example and not a full implementation. My Wordpress extension contains an improved version of that discussion embed feature, but is only designed to be used as part of the Wordpress integration.