How to create a discount code for 2-3 weeks (instead of just the first week) in Squarespace eCommerce?

Does anyone know if there's a way to add discount codes to your shop that works for the first 2-3 weeks of a subscription purchase, rather than just the first week or all throughout the subscription period?

The screenshot below are the current options, but maybe someone knows of a plugin that allows us to do this, or a workaround?

Thanks!