icloud calender: no sharing button

Hi all,

I am trying to synchronise my iPhone calender with a calender client on my Windows PC (Rainlendar). For that reason, I want to share the calender via iCloud. I found instructions, where it says to log in at icloud.com, go to Calender and click the sharing button next to the calender. But I do not have any sharing button. The space is just blank. Are there any prerequisites for it to show up?

Kind regards

Ian