How to open the stdout with Notepad++?

No, Windows does not have equivalent functionality to /dev/stdin or /dev/fd on Unixes. (It would be stdin, not stdout, from Notepad++'s point of view.) While GUI programs can read from stdin, they must be specially written to do so.

@echo off & setlocal
set f=%temp%\stdin-%random%%random%%random%.txt
more > "%f%"
start /wait notepad++ "%f%"
del "%f%"

If you don't want the batch file to "/wait" for notepadd++ to exit, you can replace the above "start /wait" line with these two lines:

start notepad++ "%f%"
REM Windows timeout won't work inside batch files. Use ping hack instead.
ping 127.0.0.1 -n 3 -w 10000 > nul