FTHR is your average heart rate during the 20 mins you’re doing the test portion of an FTP test, and is used to accurately set heart rate zones - see here for more info:
https://www.britishcycling.org.uk/knowledge/training-plans/article/izn20160310-Supporting-documents-Supporting-Document–What-to-expect-from-your-Threshold-Tests–0
If you do a Zwift FTP test while riding with a heart rate monitor, it would be trivial for Zwift to tell your your FTHR at the same time as your FTP.
Could you please implement this ASAP? Pretty please?