When you are editing an event, you can add an external link to a website. Note that the link should be with http otherwise it won't work.
So, when you fill in the description box, there is an "insert link" icon. Click on that then select the text part you want to put the link on and paste the URL that should be there.
After finishing, click on the button in the RHS corner to save the changes.
Now when you click on the blue part of text in the event description, you will be redirected to the URL you have inserted.