This feature is only available in Resolver Pro.
If you purchase a single-site or three-site license key, you can upgrade to more domains in the future.
You can upgrade via the Pricing page, just as you originally purchased Resolver Pro. In the checkout modal, click the “Enter license key” link at the top right.
This will reveal an input where you can enter your license key. Upgrading this way lets you keep the same license key, and you will receive a pro-rated discount for the time left before your license expires.