The Google Pixel Watch has a built-in optical heart-rate sensor, but the Fitbit Exercise app does not offer a way to use a Bluetooth chest strap as the heart-rate source for a workout.
With Watchletic for Wear OS, you can connect a supported Bluetooth heart-rate monitor directly to your Pixel Watch and use its readings during your workout. We use a Polar H10 in the screenshots below, but it is only one example of a supported monitor.
That is it. Press the center start button and Watchletic will use the chest strap for heart-rate data. These steps work across the Pixel Watch series, including Pixel Watch 2, Pixel Watch 3, and Pixel Watch 4.