Can I track a subdomain?

Yes - and you don’t have to do anything.

Register example.com and the same snippet works on www.example.com, blog.example.com, app.example.com and docs.example.com. All of it lands in one site. Use the same snippet on every page; don’t give a subdomain one of its own.

Visitors are shared across them, deliberately. Somebody who reads your blog and then signs up in your app is one visitor and one visit, not two of each, because the visitor identifier is built from the registrable domain rather than the host. That’s the whole point - otherwise every marketing-site-to-app journey would look like two strangers.

To see the split, use the Hostname filter. It’s in the filter menu under Page, and it breaks the same site apart by host whenever you want them apart.

Two cases where you’d want something different:

• You want the app and the marketing site kept separate. Register them as two sites - example.com and app.example.com - and give each its own snippet. Sites are unlimited, so this costs nothing. You lose the shared visitor, which is the trade.
• You’re serving on a host outside the registered domain, like a pages.dev preview or a vanity domain. That’s refused by default with the reason hostname_not_allowed. Add it under Site settings → Health with the one-click allow, or in the shields hostname allow-list.

Tracking different domains is a separate question, with a different answer.