TrainerRoad does not have plans to support heart rate (HR) based Training Stress Score (TSS) calculations.
Since HR-based TSS estimations are a function of heart-rate readings, these estimations would be subject to the myriad factors that can make HR a less-than-reliable metric (e.g. it's a lagging indicator of effort, it's largely influenced on a day-to-day basis by hydration, nutrition, sleep, stress, etc...).
We also want to avoid adding unnecessary layers of complication to our product, and in the case of HR-based TSS estimations, we don't feel the benefits outweigh the negatives.
Instead, our TSS estimator tool uses a ride's duration and a rider-reported rate of perceived exertion (RPE) for the calculation.
Admittedly, RPE is also potentially influenced by a number of factors. However, we feel the most straightforward and better solution for TSS estimation is based off of RPE only.
We know that you - the rider - know how hard, easy, or anywhere in between you pushed on a ride, and we think that trusting your perception will ultimately be the more consistent and accurate solution for TSS estimation.