Custom commands in windows Command Prompt
Create a .bat file and save it as npt.bat
In that file put the following line of code (or change it to match the path to your notepad++.exe))
@START c:\"Program Files (x86)"\Notepad++\notepad++.exe "%1"
Note that you need quotation marks around any parts of the path with spaces in it.
Now place it in whatever directory you like and add the directory to PATH in your User variables.