I have a website www.example.com and webapp app.example.com.
(Both are being monitored under the same GA4 property.)
On the website, there's a 'Login' button in the navbar that directs users to our webapp login page.
Some of our users have a habit of going to our website (example.com) and immediately clicking the login button in the navbar to navigate to our webapp's login page. (app.example.com)
This means that our Homepage has a high bounce rate, which significantly pollutes the data in reports when looking at the performance of that page.
Is there a way to filter out or exclude this type of traffic from our Google Analytics 4 Reports & explorations?
Any ideas?
Well, first of all, bounce rate is a vanity metric. It's rarely useful and it's very easy to misuse. Mostly because of all kinds of the bots bloating it up with no cookie persistence. It's also usually not used by senior analysts because it's much more useful to build channel-, campaign-, and conversion path-specific funnels and address them separately than just to look at unpainted raw bounce rate. But then once you have those funnels in place, you will never care about the general bounce rate because it will become the drop-off or abandonment rate, which can be analyzed and addressed properly.
Now back to your question. You don't need to filter anything out. What you need to do is to make sure that app.example.com sends proper tracking calls to the collect endpoint. They should have the same tid (tracking id), cid (client id) and sid (session id) parameters as the calls from the www.example.com. If that's the case, then what you've described can't contribute to the bounce rate. Read more on the definition of what bounce rate is here, but generally, it's a bounce if:
Even if you neglect to track the clicks on the log in CTA as a key event, you still should get the pageview event from the app page with the same tid,cid, and sid. If any of the ids is different, then you need to fix it.