Assuming you want to use the discussion embed feature as described by Luceos, after fixing the URL you might also need the following:
If WordPress and Flarum are found a different origins, which seems to be the case from your example code, you need to configure your server's X-Frame-Options headers. The Flarum embed extension does not add any header.
If your WordPress site uses CSP, you might also need to whitelist the Flarum domain for frames.
