Use another app’s URL scheme in Shortcuts
Shortcuts also has the ability to integrate with other apps via their URL schemes. If you want to use an app that has URL schemes but isn’t currently supported with a built-in action, you can construct the URLs manually using the URL action and launch them using the Open URLs action or the Open X-Callback URL action.
Note: Many URL schemes can be hard to implement and debug. The schemes often include a variety of parameters and require text to be URL-encoded which can be difficult to read. That said, exploring URL schemes does provide a deeper range of customisation beyond the built-in apps included in Shortcuts.
Contact the developers of your favourite apps to see if they currently use a URL scheme, or ask them to implement URL schemes so you can automate their app in Shortcuts.