Nemo Action
First, you should place the myaction.nemo_action under ~/.local/share/nemo/actions/.
Then, to make a keystroke put an underscore _ in front of the letter you want. For example in the Hello _Shortcut the s is the key. So you press the right-click and then s.
Bellow is an example of my action to open files and directories with Atom:
# Custom action for Nemo File Manager for Cinnamon.
# Adds right-click open file/files or directory with Atom.
# Place it under: ~/.local/share/nemo/actions/atom.nemo_action
# For more info: https://github.com/linuxmint/nemo/blob/master/files/usr/share/nemo/actions/sample.nemo_action
[Nemo Action]
Active=true
Name=Open in _Atom
Comment=Open %F in Atom
Exec=atom %F
Icon-Name=atom
Selection=any
Extensions=any;
Dependencies=atom;
Quote=double
Global Shortcut
If you want a global shortcut, you should use the Cinnamon Keyboard setting under the System settings, go to the second tab and add your custom shortcut.
Or, if you prefer more control over CLI you can use the dconf. For example, the bellow configuration will add the shortcut Shift><Alt>a
/org/cinnamon/desktop/keybindings/custom-keybindings/custom0/name
'Open Atom'
/org/cinnamon/desktop/keybindings/custom-keybindings/custom0/command
'atom'
/org/cinnamon/desktop/keybindings/custom-keybindings/custom0/binding
['<Shift><Alt>a']
/org/cinnamon/desktop/keybindings/custom-list
['custom0']