I have been clueless up until recently about how to install things with composer in the same way that it is done with NPM. Its amazing that something so small is so easy to do yet can still remain mysterious. Perhaps because in the grand scheme of things, it is a minute detail compared to the monstrosity of knowledge that there is to learn, when it comes to being a developer.
In any case. Here is the one liner that does this:
composer require <vendorname/packagename>
When prompted for a version just enter it or defer to dev-master.