There are many ways. Here are two easiest approaches.
I. Add customer contact (including mobile #) on Square's Customer module as shown below. While adding the customer contact, ask the customer to provide permission to send marketing related messages from your business.
2. Encourage customers to receive receipts via Text (SMS). Ask them to provide your business permission to send marketing related messages.
Follow the steps described on Send Digital Receipts to enable digital receipts and to send digital receipts.
Check out Square-Textin integration for more details.
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