freedomsupply you could embed Flarum in an iframe but it usually doesn't provide a very good experience to users, particularly on mobile.
We usually recommend installing Flarum in a subdomain or subfolder. If your website is managed with Squarespace subfolder is probably not an option but you could use an external hosting and point a subdomain to that other hosting.
If you are trying to integrate logins or memberships between the two websites, I don't think anyone has created an extension for that at this time, so if you install Flarum it would be a completely separate app in regard to user accounts.
