Kann man aus Unity in eine andere App zugreifen?

Theoretically yes.

1. If the app has an IPC interface (Inter proccess Communication), you can access it from the Unity App. However, IPC is also more intended for communication between app and background service, so you probably won’t find any documentation on an app. So you would have to make your own app that supports access to their text fields via IPC.
2. With the Win32-Api, you could enter under Windows mouse and keyboard Simulate the then move cursor and enter. However, it will be very difficult to meet something automatically.

The real question, however, is what you want to do with it. Vllt. there’s a better approach for what you want to do.