How can I use Drush in a project installed with Composer?
Basically, you need to follow steps on http://docs.drush.org/en/master/install and get Drush installed globally. It's possible to invoke Drush installed within Composer context together with Drupal 8 indicating the exact path something like /path-to-website-home/vendor/bin/drush. However, if you want Drush available anywhere in the system then get Drush Launcher installed.

Here is direct quote from http://docs.drush.org/en/master/install:


  Optional. To be able to call drush from anywhere, install the Drush
  Launcher. That is a small program which listens on your $PATH and
  hands control to a site-local Drush that is in the /vendor directory
  of your Composer project. If you skip this step, run Drush from Drupal
  root via ../vendor/bin/drush. In that case Drush's bash integration
  and custom prompt won't work.


You might also want to read How to get Drush to work with Composer and What is the right way of installing Drupal 8?