I found a solution here: https://helpcenter.websitex5.com/pl/post/7867

I created a new calendar, made an entry and then copied the HTML code:

1. Go to Settings
2. Selected my calendar
3. Scrolled to Integrate Calendar
4. Copied the "Embed Code" which starts with <iframe...
5. Created a new page in Website X5 Evo
6. Selected HTML Code and drug it to the page
7. Pasted the code <iframe... to line 1 in the HTML Code Object
8. Saved and Previewed
9. Page is blank.

What am I doing wrong? Appreciate any help!