I know this is an old question. But this may help someone who is looking for the same solution.
Method 1
You may over ride the certificate check for ALL RDP connections (use it at your own risk)
Just add a new registry key as below.
reg add "HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client" /v "AuthenticationLevelOverride" /t "REG_DWORD" /d 0 /f
Method 2
Considering if you have admin rights on the remote machine, you could actually get the crethash value from the remote machine using the below wmic command. So you could make a small batch file to get this value before you launch the mstsc and add this value in registry. I haven't included the complete batch file but thats the idea.
wmic /node:Testserver /namespace:\\root\CIMV2\TerminalServices PATH Win32_TSGeneralSetting get SSLCertificateSHA1Hash