You can create a custom command, and then map this command to the desired key. To add a custom command, you can add it to your init.coffee :
atom.commands.add 'atom-text-editor', 'custom:insert-lambda': (event) -> editor = @getModel() editor.insertText('λ')
Then you can add key mapping in keymap.cson :
'atom-text-editor': 'cmd-\\': 'custom:insert-lambda'
After rebooting or reloading the window to load a new init.coffee everything should work for you.
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