How can I use Umami analytics across subdomains?

(Already asked this question at Webmasters Stack Exchange, but I’m not sure that’s the right place for it)

I’m having trouble setting up Umami analytics with subdomains. Here’s my scenario:

At example.com, I serve my product website that advertises my service. This site includes my marketing content and ad landing pages, etc. However, serve my actual webapp over at app.example.com. I want to track users across the boundary between my app and my website, so I can, for example, see user journeys from the website to the app.

I tried just using the same Umami website ID on the root site and subdomain. As far as I can tell, though, Umami doesn’t differentiate between https://example.com/ and https://app.example.com/—they both appear as /.

I found many similar questions, but they all are specific to Google Analytics.

I’m new to Umami and analytics in general. What am I missing here? Do I need to create a custom tracker function for this usecase, sending a custom value for url that includes the subdomain? Or is this an unsupported setup?