To use Dorothy, you'll have to have a Google Cloud project on your Google Cloud account and use an API Key from that project.
To do that:- Select or create a Google Cloud project here.
- Enable billing for your project here. (Note: even though billing is enabled, you'll only be charged if you go over the free quota.)
- Enable the Text-to-Speech API here.
- Go to the Credentials page, click on Create credentials > API key.
- Copy and paste the API Key into Tasker > Preferences > Misc > Google API Key.