A dynamic context menu item like the one WinRAR createscreates is impossible to do without additional code.
You can set the (Default) value for HKEY_CLASSES_ROOT\<ProgID>\shell\mymenu
to a static string as follows:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\txtfile\shell\mymenu]
@="My Menu Entry"
[HKEY_CLASSES_ROOT\txtfile\shell\mymenu\command]
@="%SystemRoot%\\system32\\NOTEPAD.EXE %1"
This will display the following:
If you attempt to use %1 or other variablesother variables it just won't work since it'll be treated like a literal string and consequently %1 itself will show up in the menu.