composer require 'drupal/<modulename>:<version>'
Example:
composer require 'drupal/token:^1.5'
composer require 'drupal/simple_fb_connect:~3.0'
composer require 'drupal/ctools:3.0.0-alpha26'
composer require 'drupal/token:1.x-dev'
- ^1.5: maps to the latest stable 8.x-1.x release of the module.
- ~3.0: maps to the latest stable 8.x-3.x release of the module.
- 3.0.0-alpha26: maps to version 8.x-3.0-alpha26
- 1.x-dev: maps to 8.x-1.x-dev