The extension you refer to apparently only works with an Arduino
You are not using an Arduino so you need a different solution.
Some smart watches use the Wear OS (a variety of Android). To use Wear OS you need access to it's api.
and that might not be possible using App Inventor without an extension.
To get help accessing the heart rate functions of a watch; establish whether the watch is using the Wear OS; if not, provide an api to use Get to access the watch functions using Blocks.
Here is a link to the Google FIT api Google Fit | Google for Developers
See also this discussion HRM heart rate monitor
FITBit SDK Heart-rate API
GARMIN Overview | Health SDKs | Garmin Developers
Various smartwatches etc. https://www.programmableweb.com/news/10-most-popular-wearable-apis/brief/2021/11/25