to add a link

Hi Benedicte Morel Welcome (belatedly) to the Canvas Community. I see that you joined us back in May, but this appears to be your first question here.  The answer to your question depends on the type of link you want to create. If you want to create a link to another, specific item within your course, Canvas has you covered with this online guide: How do I link to other Canvas pages in a course?  If you want to link to a file in your course, that is covered here: How do I create a file link in a page in a course?  

I suspect you want to create a hyperlink to another web site, however, and this is possible by following this guide: How do I create a hyperlink in the Rich Content Editor as an instructor?  This includes YouTube videos, which will create an embedded video player, as outlined here: How do I link to a YouTube video in the Rich Content Editor as an instructor? 

So, lots of ways to do this, depending on the content you wish to link to.  The bottom line is all of these will work anywhere you see the rich content editor, which is virtually everywhere in Canvas.  

I hope this helps, Benedicte!