There really isn’t a concept of “adding” Discourse to an existing site. You create a Discourse site, and then link to it from your existing site.
Here’s a rough overview of what that process would look like:
You’d set up a Discourse site in its own environment (hosted with us or self-hosted with Digital Ocean or any other compatible hosting service).
You’d then setup the DNS with your domain registrar so your forum can be accessed at forum.mystore.com
You can optionally install and configure an OAuth2 plugin so that your customers can log in to Discourse with their Shopify account.
Then it’s a matter of including a link to your forum from your store and including a link to your store from the forum — you can also optionally theme your Discourse site to match the design of your Shopify store too.