Just received an email. Might be YMMV.

On 3‌/‌4‌/‌2‌0‌2‌1 only, at participating stores, buy a grande or larger handcrafted drink via mobile order, get a drink for free from 3‌/‌1‌0 t‌o 3‌/‌1‌2. Not valid on
purchases of hot brewed coffee or tea, Starbucks Reserve® beverages, or ready-to-drink beverages. Free drink redeemed as a coupon automatically
loaded to your Starbucks® Rewards account on 3‌/‌1‌0‌/‌2‌0‌2‌1. Coupon valid from3‌/‌1‌0‌/‌2‌0‌2‌1‌–‌3‌/‌1‌2‌/‌2‌0‌2‌1 at participating stores. Max coupon value $10 (you are responsible for any customizations exceeding this amount). Cannot be combined with other offers or discounts. Offer valid for one-time use.
Excludes alcoholic and ready-to-drink beverages.