Thanks for the answers.
I found a ready made "VO2max" workout in the application, having the format: 12 times 1+1 min, with the first minute at 135% FTP and the second minute at 55% FTP. It also offers the functionality to raise the power for the whole period. I felt it difficult but doable. Thus, next time I shall try the same format with around 5% power increase in both periods. And further increase another time, if the last interval is ok and if I fel I still have some power left.
Does it sound as a good approach for high intensity interval training?