to do that, we need to do 2 steps.
1. Configure The Configuration
First of all, we need to setup the configuration, so there's other pool for composer to look-up onto. It's the example of composer config for package of https://github.com/fiko/magento2-adminurl
composer config repositories.fiko-magento2-adminurl git firstname.lastname@example.org:fiko/magento2-adminurl.git
2. Require the package
Then, once composer config is complete, we just need to composer require package name that present on composer.json of that package. For the preceding example, we need to use this command :
composer require fiko/magento2-adminurl:dev-master