To find the file on Digital Ocean, ssh into your droplet and then run:
$ cd /var/discourse
$ ./launcher enter app
$ cd var/www/discourse/plugins
The problem with changing things that way is that your changes will be overwritten every time you update Discourse. The right way to do it is to update the plugin file that is in the repository on github and then run launcher rebuild app in your droplet.
You can do that by cloning my repository on github and making the changes you want in your cloned version. You then need to change the url that you have added to the app.yml file so that it pulls from your new repository. If this isn’t clear, just let me know 
I can also just add the domain you want to whitelist to the plugin file that is in my repository.