if your subscription includes offline licensing, you can license VNC Server using an offline license. You can find your offline license details on the Deployment page of your RealVNC account.

For all other subscription types, to license VNC Server you need to sign in using your RealVNC account credentials (email address and password).

You should have been prompted to do this when you installed VNC Server. If not, open the VNC Server menu from the taskbar and select Licensing:

Once the License Wizard opens, follow the on-screen  instructions to Sign in to your RealVNC account.