Portfolium uses a customized version of Markdown for formatting.
To add a link to the description of your entry, you can either use the ADD A URL button on the BASIC INFO screen.
You can Name the link and add the URL. When you click ADD, it will be added to your description box in the correct markdown format.
You can also just type your URL into the Description box. Make sure that your curser is where you want to add a link and type it in as seen below: