The Open a File Folder or Application action allows you to open an item and to select the application to open it with.
Click the File Chooser button to the right of the action to select an existing file on disk. Option-click on the File Chooser button to show the currently selected file in the Finder (assuming it exists already).
Select from the with pop-up which application to open the item with.
⚠️ Note that this is almost never the right action to use to launch an application. Instead, use the Activate a Specific Application action, which will both launch and activate the application. If you really want to use this action, ensure you select the Finder as the application to use to “open” the application - otherwise the system may try to open the application as a document using some other application, which is highly unlikely to be what you want to do (except in very rare circumstances, like perhaps opening an application with a compression utility to compress it for network transport).