First of all, make sure that your add-on is updated to the latest version then check which domains are going to run on your API. We’ve created an option whereby you can limit it to certain domains or available on all domains. Please read this post for more information: https://webnus.net/dox/modern-events-calendar/event-api-addon
When setting the script on your target website, you need to place the code in your editor’s text editor, not in the visual mode.
Let me know if you have any other questions in the comment area.