After an Open Application activity trying to open notepad (File Name field: C:\Windows\System32\notepad.exe) I put an Attach Window activity and whe I try to indicate on screen notepad.exe app it appears in Selector field. Attach Window activity does not find the correct app.
Ok, I will use only Open Application Activity, I did it that way (using Attach Windows activity) because in a course in UiPath RPA Academy that is the solution.
Thanks a lot @bcorrea
Keep in mind tho, that sometimes an application could at some point open a child window (like a popup), then in that case you could use an attach window inside your open application activity. Happy learning!