Each word has a Web Resources item located under the three dots. To add your own website in the Pro version, first click on the gear at the bottom of the list. Scrolling down to the bottom you will find three custom fields. Here you can enter your desired reference.
How to: Using Duden as an example. First find the website on the Internet, open it and search for any word. Then copy the URL and switch back to the application. Paste the copied URL into the URL pattern. There is also an overview of the abbreviations you can use, which you may need to use for other URLs. In this example, all you need to do is replace the word you just searched for in the URL with the capital T in curly brackets. After pressing OK, you can now look up your word on this page. If you have activated the slider, you will see your web address as a Quick Access.