The following works in PowerShell:
[void] [System.Reflection.Assembly]::LoadWithPartialName("'Microsoft.VisualBasic")
[Microsoft.VisualBasic.Interaction]::AppActivate("Readme.txt - Notepad")
[void] [System.Reflection.Assembly]::LoadWithPartialName("'System.Windows.Forms")
[System.Windows.Forms.SendKeys]::SendWait("ABCDEFGHIJKLM")
Source: http://technet.microsoft.com/en-us/library/ff731008.aspx