To create a feature in Klaviyo where customers can get a future discount by signing up for SMS, it’s a bit lengthy process but you can try these steps:

1. Create an SMS Subscribe Link: Navigate to Audience > Growth Tools in Klaviyo. Then, go to Settings > SMS > Short Link and create a new subscribe link. Choose an SMS keyword that customers will use to sign up. How to create an SMS subscribe link | Klaviyo Help Center

2. Set Up a Sign-Up Form: Create a sign-up form that includes fields for customers to enter their mobile phone number and opt-in for SMS notifications. You can use Klaviyo's form builder to create this form. How to add coupon codes to sign-up forms | Klaviyo Help Center

3. Add a Coupon Code: In the success message of your sign-up form, add a coupon block. This will display a unique coupon code to customers once they successfully sign up for SMS. You can choose to use either static or unique (dynamic) coupon codes. How to add coupon codes to sign-up forms | Klaviyo Help Center

4. Create a Flow: Set up a flow that triggers when a customer signs up for SMS. In this flow, include a message thanking them for signing up and informing them of the discount they will receive. 

5. Send an SMS Campaign: Once the customer has signed up, send an SMS campaign to confirm their subscription and provide them with the coupon code. Use the template tag {% coupon_code 'CouponName' %} to include the coupon code in your message. How to use unique coupon codes in SMS and MMS messages | Klaviyo Help Center
   
   Let me know if this works!