The keyboard shortcut for “Open With” in Finder

Both return and command + O work to open files in my version of coda.

Is your enter key only an enter key or combined?

It doesn't look like that plug in changes key bindings so you might have to search elsewhere for something that is altering the standard behavior. Other tweaks or software possibly?

Well, here you go with an AppleScript for that.

First, create the AppleScript:

1. Open Automator
2. Create a Quick Action
3. Set the input to no input
4. Drag and Drop the Run AppleScript workflow element onto the grey space.
5. Paste the code from below into the AppleScript
6. Save the workflow as Create new file
   If you have iCloud Drive activated, make sure you are saving the file under Library/Services/ in your Home Folder.

try
  tell application "Finder" to set the this_folder ¬
   to (folder of the front window) as alias
on error -- no open folder windows
  set the this_folder to path to desktop folder as alias
end try

set thefilename to text returned of (display dialog ¬
 "Create file named:" default answer "filename.txt")
set thefullpath to POSIX path of this_folder & thefilename
do shell script "touch \"" & thefullpath & "\""

To add it as a shortcut:

• Go to System Preferences -> Keyboard -> Shortcuts -> Services
• Scroll down until you find the Service Create new file
• Assign a shortcut to it by clicking on the right side none, which turns into a Add Shortcut.
  Click the Button and type the shortcut you wish to use.
  I use ⌘ Command+⌥ Option+N