Nothing to configure on Cloudflare. Make sure your webserver is working properly and you have a valid certificate on your server and it will work out of the box.
I am not overly familiar with Wordpress. Generally you dont need to add the API key at all, that is pretty optional. I believe it only is required if you want to manage your Cloudflare account from our Wordpress control panel.
If you have two WordPress installations under the same domain (example.com), regardless of whether the second is installed on a subdirectory or subdomain, you may add the API key to both installations of WP Rocket for your convenience, as you will then be able to ask the plugin to perform whatever CF tricks it is capable of from the back end of either one.
However, AFAIK if the plugin performs a “Purge All” operation, it will purge the CF cache from both of them, no matter which installation asked for it. You may want to check the plugin documentation to see whether it can do a partial purge based on the subdomain or subdirectory it is installed on.