To request consent from your customers to receive SMS messages,
You must follow these steps on your Shopify store:
Go to Settings.
Navigate to Payment screen.
Select Marketing options and check the following:
You can also use the following link: https://admin.shopify.com/store/YOURSTORENAME/settings/checkout replacing 'YOURSTORENAME' with the name of your store.
Afterward, in the checkout process, customers will have the following option:
Customers who check this box will have the 'YES' value for the 'Accepts SMS attribute'.
Additionally, in each campaign's sending, you can offer users the option to unsubscribe through an unsubscribe link.