https://www.tawk.to/knowledgebase/plugins-and-modules/magento-2-integration/

===========================================

[root@sixteen public_html]# php composer.phar require tawk/widget

Authentication required (repo.magento.com):
  Username:  my_magento_public_key
  Password: my_magento_private_key

Do you want to store credentials for repo.magento.com in /root/.composer/auth.json ? [Yn] y

[InvalidArgumentException]
Could not find package tawk/widget at any version for your minimum-stability (alpha). Check the package spelling or your minimum-stability

require [--dev] [--prefer-source] [--prefer-dist] [--no-progress] [--no-suggest] [--no-update] [--no-scripts] [--update-no-dev] [--update-with-dependencies] [--ignore-platform-reqs] [--prefer-stable] [--prefer-lowest] [--sort-packages] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--] [<packages>]...

==========================================

{
  "name": "tawk/widget",
  "description": "Tawk live chat widget",
  "type": "magento2-module",
  "version": "1.0.0",
  "license": [
    "OSL-3.0"
  ],
  "require": {
    "php": "~5.6.0|7.0.2|7.0.4|~7.0.12"
  },
  "autoload": {
    "files": [ "registration.php" ],
    "psr-4": {
      "Tawk\Widget\": ""
    }
  }
}

http://webtips.krajee.com/setting-composer-minimum-stability-application/

https://igor.io/2013/02/07/composer-stability-flags.html