Where can I access the API documentation?

Head over to your dashboard. By clicking on your name and email address, a menu opens and there you can see the API; or you can simply click here for a shortcut: https://app.watchthem.live/account-api

How can I track my websites’ subdomains?

You’d need to add every subdomain as a separate website. Keep in mind that there’s no need to add your website’s subfolders as separate websites (website.com/subfolder) Only subdomains should be added as separate websites (subdomain.website.com)