To create a link from Custellence to a place outside the tool, you simply grab the URL in the browser window of the chosen application, and then paste it in the Custellence card.

Here is an example from a Trello board I want to refer to from a card in my customer journey map. I copy the URL...

...and just paste it in my card in Custellence. It will be shown as a clickable link. You can also write Link: [ insert URL ]