It depends, if you have just the starter version there’s no license required.
If you have the starter with our sending service (the 500 free credits per month), this has a license key used by our API to manage your account.
Note that you don’t need to renew it, it should renew automatically since it’s free.
I do indeed have a “Starter” plan, and as previously mentioned,
I would also point out that there is no option to enter a license key—whether free or paid—which I find strange.
For pro or customer support, please contact the developer on their site. This includes pre-sales information. This includes license keys.
As the developer is aware, customers may not be supported on this site.
https://wordpress.org/support/guidelines/#do-not-post-about-commercial-products