How to get service account JSON file for authentication

Go to Google Cloud Console and select the project you want to add text to speech API

Open sidebar and select APIs and Services then Library

Search for text to speech and select Cloud Text-to-Speech API

Enable Cloud Text-to-Speech API. To do this you need to have enabled billing

Open sidebar go to APIs and Services and select Credentials

Click Create Credentials and select Service account

Add service account name select Owner as role and click done

Select created Service account

Go to keys tab and click Add Key button

Then select JSON and click Create. Then it will ask you to save Service Account JSON file. You can upload it in settings page of WPTTS to authenticate