Go to My Pass, tap on your pass and then tap on the 3 dots at the top right. There should be an option to deactivate.

Then, don't activate the pass until your start day. There's no need and no advantage, only a possible disadvantage. You can only cancel activation until 23:59 before validity starts and if something unexpected happens after that (e.g. you wake up ill), then you can't cancel anymore and you loose travel days. The same is true for activating travel days on a flexipass.

Awesome!! Thanks for your quick help!!