There is a "Notes" section in some versions of the Starbucks app where you can add extra instructions for your order. Write down your preferred milk in these notes in a polite way, hoping that the barista will see them and follow your lead. It is important to keep in mind that this method does not guarantee success because it depends on the barista following your notes.
If you want to be more proactive, you could call the store before you place your order. This step is especially helpful if you're not sure if an ingredient will be available or if you want to be sure you'll get the milk you want. Tell the store staff exactly what you want during the call to make sure they have the ingredients they need and can accommodate your request. This proactive step is meant to make sure everything goes smoothly and avoid any disappointments upon your arrival.
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