If you want an OTP every time someone buys on your card, you need to enable the authentication function.
You can do this when you add a funding source to a Xare card. Here's how:
Step 1: Tap Add Funding source on the card you want to fund.
Step 2: Fill in all required bank card information.
Step 3: At the bottom of the app screen, you will see the ‘Enable authentication’ toggle button. Tap to enable.
Step 4: Tap "Add your card" to save.