For information on setting up a RealVNC account and downloading/installing VNC Connect, see either:
Home or Professional subscription
There is no license key. Instead, you license VNC Server (running on each computer you want to control) by signing in to it 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:
Follow the instructions to Sign in to your RealVNC account.
You can license VNC Server in the same way as above, by signing in using your RealVNC account credentials on the remote computer you want to control.
However, you also have a license key, available from the Deployment page of your RealVNC account. You can use this to license VNC Server in bulk, offline, or remotely. Follow these instructions: https://www.realvnc.com/en/connect/docs/faq/license-key.html
Additionally, you have a legacy licence key for VNC 5.x, so you can remotely access old platforms that are no longer supported by VNC Connect. This is also available from the Deployment page of your RealVNC account.