Install ACF PRO

Declare a new repository

Edit composer.json

"repositories": [
  {
    "type": "package",
    "package": {
      "name": "elliotcondon/advanced-custom-fields-pro",
      "version": "5.9.5",
      "type": "wordpress-plugin",
      "dist": {"type": "zip", "url": "https://connect.advancedcustomfields.com/index.php?p=pro&a=download&k={%ACF_PRO_KEY}&t={%version}"},
      "require": {
        "ffraenz/private-composer-installer": "^5.0",
        "composer/installers": "^1.4"
      }
    }
  },
  {
    "type":"composer", "url":"https://wpackagist.org"
  }
]

Edit your .env file to add your ACF Pro Key

ACF_PRO_KEY=Your-Key-Here

Then require with composer

$ composer require elliotcondon/advanced-custom-fields-pro