FREE Sonic Drive-In Slush April 26, 2016 by SMFM Leave a Comment The links in the post below may be affiliate links. Read the full disclosure Here. Download the Sonic Drive-In app for IOS or Android and set up your account. When you do, you’ll receive a FREE slush – no purchase required! You should receive the offer automatically after setting up your account. Claim your FREE Slush by clicking “redeem” and showing the coupon to your carhop at any participating Sonic Drive-In!