How do I add an extra field to my portfolio projects ?
— Learn how to build your portfolio page.
You can add portfolio details yourself again -> Dashboard -> Theme options -> Portfolio -> Details -> Portfolio Details as on the screenshot below.

After that you can set the value for this field for each portfolio project via: Edit Portfolio -> Page Options -> Details as on screenshot below.

Here you can enter a clickable url that opens a new page, if you do not enter a portfolio detail, the label will not appear on the page either.