Nucleus Card's can be set to directly link to an external URL and this URL can be customized within your Card Settings.
To access Card Settings, click the gear icon at the top of the page while in the Nucleus Card Editor.
Upon clicking the gear icon, you'll notice a new set of options appears in the sidebar. Here, you can begin customizing your Card Settings - including a Card's external linking URL.
Paste the external URL you wish to directly link your card to into the field provided.
If you wish to have your Card link open within a new tab in a user's browser, make sure to select the checkbox option to allow for that. Leaving this box un-checked will mean your Card link will open within a user's existing Nucleus tab.
When you're ready to to update your Card and commit these changes, click the 'Update' button at the top of the page.