By default new cards are added to the top of a box or stack. But if you want new cards you add to appear at the bottom of a stack, open your box or stack and click the three dots icon in the top menu bar.

Here, as well as being able to choose a sort sequence for box or stack, you'll see an option to add new cards to the top or bottom of the pile.

Choose New cards added to the bottom and close the page. Next time you add a card to the box or stack it will appear beneath the rest of the cards you've already added – useful if you're adding new cards to build on a story or presentation flow.