where do I get the secret property for creating webhook for azure speech to text service

Question

If you look at the properties key,secret value is SecretUsedToCreateHMACHexDigestThatYouWillFindIn where do i get this value from ?

{
  "displayName": "TranscriptionCompletionWebHook",
  "properties": {
    "secret": "SecretUsedToCreateHMACHexDigestThatYouWillFindIn\"X-MicrosoftSpeechServices-Signature\"HeaderWhenBeingCalledBack"
  },
  "webUrl": "https://contoso.com/call/me/back",
  "events": {
    "transcriptionCompletion": true
  },
  "description": "I registered this URL to get a POST request for each completed transcription."
}