Allow use of the API key in the additional headers configuration.

Closes #3524559

src/Plugin/AiProvider/AzureProvider.php

@@ -270,6 +270,8 @@ class AzureProvider extends AiProviderClientBase implements
 
     // Also check for extra headers.
     if (!empty($data['extra_headers'])) {
+      // String replacement for @apikey.
+      $data['extra_headers'] = str_replace('@apikey', $this->apiKey, $data['extra_headers']);
       foreach (explode("\n", $data['extra_headers']) as $header) {
         // Tokenize the header.
         if ($this->tokenTree) {
@@ -634,7 +636,7 @@ class AzureProvider extends AiProviderClientBase implements
     $form['model_data']['advanced_settings']['extra_headers'] = [
       '#type' => 'textarea',
       '#title' => $this->t('Extra headers'),
-      '#description' => $this->t('Sometimes you might need extra headers for Azure, you can add it here. One header per line in key:value format. If Token module is installed, user tokens are available here.'),
+      '#description' => $this->t('Sometimes you might need extra headers for Azure, you can add it here. One header per line in key:value format. If Token module is installed, user tokens are available here. The API key can be substituted with <em>@apikey</em>'),
       '#attributes' => [
         'placeholder' => "Authorization:Bearer 123\nContent-Type:application/json",
       ],