Then under it, create a new key called "command", change it data to your path to emacs, plus a "%1" at the end.
If you want to add Emacs to just a specific type of file, for example, ".org" file for Orgmode, it's a little more work.
Create a new key ".org" under HKCR, give it data value "Orgmode.File". Then create a new key "Orgmode.File" under HKCR. Under it, create key "Shell", under which create key "Emacs", under which create key "command", change its data to emacs path plus "%1".
If you want to add Emacs to directory context menu, create the same keys under HKCR/Folder.
Of course, all these apply to any programs, not just emacs.