If your subscription includes offline licensing, you can license RealVNC 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 RealVNC 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 RealVNC Server. If not, open the RealVNC 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.