Training Peaks uses your anaerobic threshold to calculate TSS (not your aerobic) so you’ll want to enter the heart rate that represents the bottom of Zone 4. If you use your aerobic threshold, the TSS calculations in Training Peaks will be exaggerated.
If you’ve had a lab test, I would ignore the Training Peaks recommendations (and notifications) and go by the results of your test. Training Peaks formulas will be generic, but your test will be specific to you.
I hope that helps.
P.S. If it’s unclear from your test where the bottom of Zone 4 is, attach the test in a follow-up post, and I can take a look at it.