diff --git a/.github/workflows/notification.yaml b/.github/workflows/notification.yaml
deleted file mode 100644
index 94ef458..0000000
--- a/.github/workflows/notification.yaml
+++ /dev/null
@@ -1,21 +0,0 @@
-on:
-  release:
-    types: [published]
-
-permissions:
-  contents: read
-
-jobs:
-  notify:
-    name: Notify
-    runs-on: ubuntu-latest
-    steps:
-      - name: Send release notification
-        uses: slackapi/slack-github-action@485a9d42d3a73031f12ec201c457e2162c45d02d # v2.0.0
-        with:
-          webhook: ${{ secrets.SLACK_WEBHOOK_URL }}
-          webhook-type: webhook-trigger
-          payload: |
-            repo: "${{ github.repository }}"
-            url: "${{ github.event.release.html_url }}"
-            version: "${{ github.event.release.tag_name }}"
diff --git a/composer.json b/composer.json
index c61cf6c..bd0d6f3 100644
--- a/composer.json
+++ b/composer.json
@@ -20,6 +20,9 @@
 			"homepage": "https://github.com/sumup"
 		}
 	],
+	"scripts": {
+		"cs": "php-cs-fixer fix --dry-run --diff"
+	},
 	"require": {
 		"php": "^5.6|^7.0|^8.0"
 	},
diff --git a/examples/simple.php b/examples/simple.php
new file mode 100644
index 0000000..631efba
--- /dev/null
+++ b/examples/simple.php
@@ -0,0 +1,13 @@
+<?php
+
+/*
+ * Run with `php simple.php`, don't forget to set SUMUP_API_KEY anv variable with your API key
+ */
+
+require __DIR__ . '/../vendor/autoload.php';
+$sumup = new \SumUp\SumUp([
+    'api_key' => getenv('SUMUP_API_KEY'),
+]);
+
+$dba = $sumup->getMerchantService()->getDoingBusinessAs();
+print_r($dba->getBody());
diff --git a/src/SumUp/Application/ApplicationConfiguration.php b/src/SumUp/Application/ApplicationConfiguration.php
index 3bd2f36..40a0701 100644
--- a/src/SumUp/Application/ApplicationConfiguration.php
+++ b/src/SumUp/Application/ApplicationConfiguration.php
@@ -105,6 +105,13 @@ class ApplicationConfiguration implements ApplicationConfigurationInterface
      */
     protected $customHeaders;
 
+    /**
+     * The API key for authentication.
+     *
+     * @var string|null
+     */
+    protected $apiKey;
+
     /**
      * Create a new application configuration.
      *
@@ -115,6 +122,7 @@ class ApplicationConfiguration implements ApplicationConfigurationInterface
     public function __construct(array $config = [])
     {
         $config = array_merge([
+            'api_key' => null,
             'app_id' => null,
             'app_secret' => null,
             'grant_type' => 'authorization_code',
@@ -129,6 +137,7 @@ public function __construct(array $config = [])
             'custom_headers' => []
         ], $config);
 
+        $this->apiKey = $config['api_key'];
         $this->setAppId($config['app_id']);
         $this->setAppSecret($config['app_secret']);
         $this->setScopes($config['scopes']);
@@ -273,6 +282,16 @@ public function getCustomHeaders()
         return $this->customHeaders;
     }
 
+    /**
+     * Returns the API key if set.
+     *
+     * @return string|null
+     */
+    public function getApiKey()
+    {
+        return $this->apiKey;
+    }
+
     /**
      * Set application ID.
      *
@@ -282,8 +301,8 @@ public function getCustomHeaders()
      */
     protected function setAppId($appId)
     {
-        if (empty($appId)) {
-            throw new SumUpConfigurationException('Missing mandatory parameter app_id');
+        if (empty($appId) && empty($this->apiKey)) {
+            throw new SumUpConfigurationException('Missing mandatory parameter app_id or api_key');
         }
         $this->appId = $appId;
     }
@@ -297,8 +316,8 @@ protected function setAppId($appId)
      */
     protected function setAppSecret($appSecret)
     {
-        if (empty($appSecret)) {
-            throw new SumUpConfigurationException('Missing mandatory parameter app_secret');
+        if (empty($appSecret) && empty($this->apiKey)) {
+            throw new SumUpConfigurationException('Missing mandatory parameter app_secret or api_key');
         }
         $this->appSecret = $appSecret;
     }
diff --git a/src/SumUp/Application/ApplicationConfigurationInterface.php b/src/SumUp/Application/ApplicationConfigurationInterface.php
index d66d03d..2be4592 100644
--- a/src/SumUp/Application/ApplicationConfigurationInterface.php
+++ b/src/SumUp/Application/ApplicationConfigurationInterface.php
@@ -92,4 +92,11 @@ public function getForceGuzzle();
      * @return array
      */
     public function getCustomHeaders();
+
+    /**
+     * Returns the API key if set.
+     *
+     * @return string|null
+     */
+    public function getApiKey();
 }
diff --git a/src/SumUp/Services/Authorization.php b/src/SumUp/Services/Authorization.php
index 9e301d7..e6d5053 100644
--- a/src/SumUp/Services/Authorization.php
+++ b/src/SumUp/Services/Authorization.php
@@ -57,6 +57,16 @@ public function __construct(SumUpHttpClientInterface $client, ApplicationConfigu
      */
     public function getToken()
     {
+        if (!empty($this->appConfig->getApiKey())) {
+            return new AccessToken(
+                $this->appConfig->getApiKey(),
+                'Bearer',
+                null,
+                [],
+                null
+            );
+        }
+
         $accessToken = null;
         if (!empty($this->appConfig->getAccessToken())) {
             $accessToken = new AccessToken(