You want to switch from a subdomain (abcd.example.com) to a subfolder (example.com/abcd).
Change config.json to reflect the new url
"url": "http://forum.example.com" -> "url": "http://example.com/forum"
Use nginx to reverse proxy requests by adding a new location block for /forum
@g0st Pushed an update so that the topic cards use the topic thumbnail if available, and added a few extra options in the widget config.
Feel free to open any other feature requests in the github issues.