To disable credentials prompt for RDP client, add below line to a connection file (or to default.rdp file in c:\users\username\Documents for global change)
enablecredsspsupport:i:0
This option disables prompting for credentials, but cannot be used with
Network Level Authentication option enabled for RDP.
NLA is available on Windows Vista and newer, and requires credentials to be provided by the client before a session is created on the server side.
LeoStar IT 