If I understand correctly, you want PowerShell to send the ENTER keystroke to some interactive application?
$wshell = New-Object -ComObject wscript.shell;
$wshell.AppActivate('title of the application window')
Sleep 1
$wshell.SendKeys('~')
If that interactive application is a PowerShell script, just use whatever is in the title bar of the PowerShell window as the argument to AppActivate (by default, the path to powershell.exe). To avoid ambiguity, you can have your script retitle its own window by using the title 'new window title' command.
A few notes:
{ENTER}, though they're not identical - that's the keypad's ENTER key. A complete list is available here: http://msdn.microsoft.com/en-us/library/office/aa202943%28v=office.10%29.aspx.Sleep 1 statement is to wait 1 second because it takes a moment for the window to activate, and if you invoke SendKeys immediately, it'll send the keys to the PowerShell window, or to nowhere.Sometimes wscript.shell's SendKeys method can be a little quirky, so if you run into problems, replace the fourth line above with this:
Add-Type -AssemblyName System.Windows.Forms
[System.Windows.Forms.SendKeys]::SendWait('~');