diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index 9b76f61..4da42b4 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -380,7 +380,7 @@ async function _synthOnPremMicrosoft(logger, {
   text,
   filePath
 }) {
-  const {use_custom_tts, custom_tts_endpoint_url} = credentials;
+  const {use_custom_tts, custom_tts_endpoint_url, api_key} = credentials;
   let content = text;
 
   if (use_custom_tts && !content.startsWith('<speak')) {
@@ -404,7 +404,8 @@ async function _synthOnPremMicrosoft(logger, {
     const post = bent('POST', 'buffer', {
       'X-Microsoft-OutputFormat': trimSilence ? 'raw-8khz-16bit-mono-pcm' : 'audio-16khz-32kbitrate-mono-mp3',
       'Content-Type': 'application/ssml+xml',
-      'User-Agent': 'Jambonz'
+      'User-Agent': 'Jambonz',
+      ...(api_key && {'Ocp-Apim-Subscription-Key': api_key})
     });
     const mp3 = await post(custom_tts_endpoint_url, content);
     return mp3;