Issue #3272538 by m4olivei, markturnip, e0ipso: No configuration form

modules/api_proxy_example/api_proxy_example.info.yml

+name: API Proxy Example
+description: Illustrates the implementation of an HTTP API plugin.
+core: 8.x
+core_version_requirement: ^8 || ^9
+type: module
+dependencies:
+  - api_proxy:api_proxy

modules/api_proxy_example/src/Plugin/api_proxy/Example.php

+<?php
+
+namespace Drupal\api_proxy_example\Plugin\api_proxy;
+
+use Drupal\api_proxy\Plugin\api_proxy\HttpApiCommonConfigs;
+use Drupal\api_proxy\Plugin\HttpApiPluginBase;
+use Drupal\Core\Form\SubformStateInterface;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * The Example API.
+ *
+ * @HttpApi(
+ *   id = "api-slug",
+ *   label = @Translation("Example API"),
+ *   description = @Translation("Proxies requests to the Example API."),
+ *   serviceUrl = "https://api.example.org/v1",
+ * )
+ */
+final class Example extends HttpApiPluginBase {
+
+  use HttpApiCommonConfigs;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addMoreConfigurationFormElements(array $form, SubformStateInterface $form_state): array {
+    $form['auth_token'] = $this->authTokenConfigForm($this->configuration);
+    $form['more_stuff'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Extra config'),
+      '#default_value' => $this->configuration['more_stuff'],
+    ];
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function calculateHeaders(array $headers): array {
+    $default_headers = parent::calculateHeaders($headers);
+    // Modify & add new headers. Here you can add the auth token. Refer to your
+    // APIs documentation for expected auth format.
+    return array_merge(
+      $default_headers,
+      [
+        'authorization' => ['Basic ' . base64_encode($this->configuration['auth_token'] . ':' . $this->configuration['more_stuff'])],
+        'accept' => ['application/json'],
+        'content-type' => ['application/json'],
+      ]
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function postprocessOutgoing(Response $response): Response {
+    // Modify the response from the API.
+    // A common problem is to remove the Transfer-Encoding header.
+    // $response->headers->remove('transfer-encoding');
+    return $response;
+  }
+
+}