Per step 8: If applicable, repeat steps 3-7 for the Mobile Key.

You tap on the mobile key and then tap Public Keys.

Alternatively, you can set up a watch only wallet in sparrow quite easily with the descriptor.

To view and save the wallet descriptor for your vault in the Casa app, on the "Receive BTC" screen just tap on the "..." options button in the upper right and you'll see the toggle between viewing a single address and the full wallet descriptor.