Have you read the plugin description ;)?
Depending on browser origin policy restrictions of frontend users it could be possible to embed only pages inside gpEasy root!
So it's not recommended to use it with an url outside gpEasy root. you can try it, it will maybe work on your browser but there's no guarantee that's the same on other browsers like the current one with higher security levels.
So the plugin asks for a relative path like /forum (will be http://www.your-domain.tld/forum).
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