First, run this command in the console. Make sure you are in the same folder where the forum is saved.
Then you follow the first steps as described by Justoverclock. Go to Extiverse, create a token and adjust the command replacing Pasteyourtokenhere with your token. Then enter this command in the console as well.
Once you've done that, enter the command
composer require **justoverclock/discord-widget:"*"
Now refresh your page with F5 and you should find the extension in the admin panel.