In Server 2012, you could select the SSL sertificate for the RDS server in the RDP-TCP properties. These have been removed in Server 2012.
Run the following command in an elevated cmd on every server in your RDS-farm:
wmic /namespace:\\root\CIMV2\TerminalServices PATH Win32_TSGeneralSetting Set SSLCertificateSHA1Hash="Thumbprint"
Where “Thumbprint” is the thumbprint of your SSL certificate, without the spaces. If you’re using a self-signed certificate, remember it must be imported on each server first.