cmdkey and Mstsc

cmdkey /generic:TERMSRV/"computername or IP" /user:"username" /pass:"password"

Then call RDP connection using

mstsc /v:"computer name or IP"

If you want to delete the credentials run

cmdkey /delete:TERMSRV/"Computer name or IP"

Remember to remove ""

http://stackoverflow.com/questions/11296819/run-mstsc-exe-with-specified-username-and-password

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Re-posted as an answer: Found an alternative (Tested in Win8):

cmdkey /generic:"<server>" /user:"<user>" /pass:"<pass>"

Run that and if you run:

mstsc /v:<server>

You should not get an authentication prompt.

http://stackoverflow.com/questions/14481882/login-to-remote-using-mstsc-admin-with-password