How to insert internet link to a card?

Click on the topic in your teaching plan. Press the red + sign in the bottom right corner. Select "text card" from the slide options.

Click inside the text field. Editing options display in a top menu. Select the link icon. Insert the link and press the green tick icon to save changes.

A link can be inserted into other types of cards too. Just click into the edit field to display the edit options including the link option.
A link can be inserted into an existing card too. Edit the card and continue respectively.