I could not get plugins to load using the external url method, but loading plugins locally works.
You need to create a new plugin
mkdir -p ~/.c9/plugins/myPlugin/snippets;
cd ~/.c9/plugins/myPlugin/snippets;
printf '# scope: javascript\nsnippet test\n\tif (${1:true}) {\n\t\t${2}\n\t}\n\t$0' > javascript.snippets;
echo '{"name":"myPlugin", "plugins": {}}' > ../package.json
Then open your init script (click on AWS Cloud9 > Open Your Init Script in the menu bar) and add code for loading the plugin
services["language.complete"] = services["languageComplete"];
services.pluginManager.loadPackage([
"~/.c9/plugins/myPlugin/package.json",
])
To add more snippets edit ~/.c9/plugins/myPlugin/snippets/javascript.snippets
NOTE: snippet file needs to be indented with tabs not spaces
I pieced this together from the two links below.
How to load plugins locally: https://community.c9.io/t/snippets-not-working-in-c9/19215/3
Fix for AWS not loading snippets: https://forums.aws.amazon.com/thread.jspa?threadID=299949&tstart=0