specifying project in a TestExecute run

I'm assuming you've read this article?

https://support.smartbear.com/testcomplete/docs/working-with/automating/command-line-and-exit-codes/command-line.html

Because, all you need to do is pass in the project suite as the file name and then the project name.  That is, so long as the project is part of the project suite.  Something like

"C:\Program Files\SmartBear\TestComplete 12\Bin\TestComplete.exe" "C:\My Projects\MySuite.pjs" /r /p:MyProj /e /SilentMode

The above example is taken straight from the help topic.  This will load the project suite and then execute the project indicated.

Now, just one note... the project must be part of the project suite.  If you're trying to run a project and have it report under a project suite to which it is not attached, this will not work.  But the above code line should do what you're asking given any project within a given project suite.