Be aware, that only works for your homepage and not specific items in your forum, which you may also want to seek out in the future, such as threads or 3rd party addons.
You will need to do a template modification and add arg-imageUrl="" to the macro template metadata_macros with the name metadata. This way, you can have a specific thread if you want to have its own image other than the site's image (which can draw in more users as it's more relevant to the link title/description and can be a larger image, meeting Twitter [large card] or Facebook recommended sizes, to not be shared as a small image of just your forum). Doing this will drastically improve the click-through rate.
That said, these are more complex edits and should only be done on a dev installation or you could mess some major stuff up.