Portfolium uses a customized version of Markdown for formatting.
To add a link to the description of your entry, you can use the Insert link into description option located below the description box.
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: