According to the FAQ: https://www.sidefx.com/faq/question/how-do-i-set-up-the-houdini-environment-for-command-line-tools/ [www.sidefx.com]

When installed with Launcher:

Select Command Line Tools from the little triangle to the right of 'Launch' button on the Houdini section of Launcher.

But this method 1) needs to use the GUI first 2) opens a new window.

I'd like to use Houdini Command Line in an existing terminal window like a shell. Like Git bash or Powershell. Is it possible? (I'm on Windows)

If you're running something like bash, you can cd into the Houdini installation directory and run source houdini_setup
If you're using Powershell or terminal, you can cd into the bin directory in the Houdini installation directory and run hcmd.exe