To add money to a Linked card please press the "Cards" option at the bottom of the app screen. You'll be able to see all your cards including Linked Cards there. Please select "Add Money" under the card you wish to top up. On the next screen you can select "Top up" or exchange. If you continue with "Top up" you'll be able to add money to your Linked Card directly from your bank account. If you'd prefer to move money across from your personal card, please choose "Exchange" and you'll be able to transfer money from your personal card.
Please note that Linked Cards can only be managed from the mobile app.