In custom training have a "Max Effort" option

IN the zwift trainer you can set zones, intervals etc. All of these require you to set specific power zones (Apart from free ride).

It would be good to have a “Max Effort” option where no power is set but it’s highlighted in red like z6 efforts. You should just be able to set the time. 

I use Free Ride for those and put a text note in the workout to go hard.