Let's say there is a pre-existing Android app on my phone that has a button on its home screen that I can click. I know there is a way to launch that app from an app I am creating, but is there a way to click the button as well automatically? Is there a parameter I can pass in, and how would I go about using it?
You can't have your apps push a button on another app. That would be a giantic security leak.
However, the other app might have options on how to call it. It will depend on that other app. You would call the other app with an Intent, then use putExtra to provide extra parameters.