I asked for help to the SS help center and here is their response - so disapointing.
Are Forums easy to integrate with SS ? And how to do that? Since this would need to be done using custom code, while you're welcome to use custom code to modify your site, we highly recommend that you have coding knowledge before attempting custom code solutions. Due to their complex nature, we're unable to offer assistance or troubleshooting for code modifications. Adding code to your site may cause unwanted results when interacting with our platform, templates, or updates we release. To learn more, visit our Custom code FAQ: https://support.squarespace.com/hc/en-us/articles/216107348 Our forum is another great resource: http://forum.squarespace.com The forum is an online community of Squarespace users who help each other with more complex, code-related solutions. If you can't find an existing post that answers your question exactly, I'd recommend creating an account and posting your question. By experience could you tell me what forum work WELL integrated with SS ? For security reasons, I don't want to have to download any forum software to my Mac. Also the free software seems too good to be true. We wouldn't be able to say if any forum would work well, since you'll need to modify the site with custom code we can't guarantee it's functionality within Squarespace. I hope you found the information provided today helpful, if you have any further questions or concerns please feel free to contact us. Our support team is available 24/7 to help.
Ahahaha !