One way of setting a new power PB is when you raise your FTP, chances are your 5 min power for example might be a PB for you, assuming you’ve never reached those kinds of numbers before in your training history.
Another way is to go a test your best ‘x’ minute power by riding flat out for that duration of time.
You can try longer intervals or shorter rest periods which is what I prescribe for my athletes but they’re not following a TR plan. I guess you need to decide whether you’re happy following the TR plans for you fancy mixing your own into the equation.