.env

SOME_API_KEY=12345

some-api-config.php

<?php

return [
  'key' => env('SOME_API_KEY');
  ];

SomeApiRepository.php

<?php

class SomeApiRepository {
  
  protected $key;
  
  function __construct ($key) {
    $this->key = $key;
    // add any other dependencies here, like Guzzle
  }
  
  /**
   * Abstract stuff to helper protected methods as usual
   */
  function getSomeInfo() {
    return $this->httpFetcherDoohikie->grabSoAndSo();
  }
  
}

SomeApiServiceProvider.php

<?php

class SomeApiServiceProvider extends ServiceProvider {
  
  public function register() {
    
    $this->app->bind(SomeApiRepository::class, function ($app) {
      return new SomeApiRepository($app['config']->get('some-api-config.key'));
    });
    
  }
  
}