On the side of the website you have the drop down menu (3 horizontal bars on mobile in the top left next to the grow diaries website banner) press it and it will open with loads of options.
One of the options you can click is called "Contests" open it and just search until you find a Dutch Passion contest that is active.
If you can't find any competitions currently for example, It's because they have none. Check back in a few weeks and they might have put up a new one. They do competitions all the time.
When you sign up for the competition, this needs to be done after you have finished the harvest of a plant that meets the contest specifications such as strain type, indoor or outdoor grow. Once your completed diary meets the contest specifications you can enter using said diary and then when the competition finishes, if you win they will contact you via the email address you have registered using your grow diaries account and send you the prize for whichever place you get.
Good luck with your grow =)