I tried to use Microsoft Layout Creator, though I do not know how to show the numeric keypad in this program. I tried editing the source klc file, but I did not find documentation about the language.
I am using US-International layout, and would like to press the "num pad minus" (-) + shift return underline (_)
I am use windows 10
I'm not aware of a native way to do this, but there are a variety of key mapping applications available which would allow this.
One popular option is AutoHotkey, which uses small scripts to map keys or run automations. Creating a script with the following code would accomplish your goal:
+NumpadSub::_
In the syntax of AHK the :: separates a hotkey string from a command/commands to execute. The + indicates use of the Shift key, and NumpadSub is a special name for the - key on the number pad. After the pair of colons is just an underscore, indicating that the underscore character should be typed.
If you enter this into a text document saved with the .ahk extension, AutoHotkey will be able to run it and the key mapping will be available as long as the script is running. You can cause the script to launch with windows by placing it or a shortcut to it in the startup folder:
%appdata%\Microsoft\Windows\Start Menu\Programs\Startup
Or you can launch the script manually when needed. For more information on AutoHotkey check their extensive documentation.
