I want a Siri shortcut to open the Messages app showing the thread with myself. So I have to use the Open App action, and open Messages. But is it possible to pass variables to the action so that Messages is opening the correct thread? I tried passing my contact to the App, text with my name and telephone number, but it didn't work.
It is not possible to pass variables to an App opened by Siri.
Since there is no predefined action available that opens a specific thread in the Messages App, it is not possible to create a shortcut that fits your request.