In case it helps, the term to Google here is “Modal Dialog”. A Modal window pops to the front, and (optionally) can prevent switching to any other application.
Notepad itself does not have that functionality, but it could be done with a VBScript dialog box such as the tutorial at https://www.tutorialspoint.com/vbscript/vbscript_dialog_boxes.htm
There’s probably a PowerShell way too, if that’s your preference.