Create a new project (or pick an existing one) from the top bar.
(You'll need to enter a Visa/Mastercard to activate billing.)
Go to APIs & Services → Libraryfind Cloud Text-to-Speech APIand click Enable
Go to APIs & Services → Credentials → Create Credentials → API key
Copy the key and paste it into the Google TTS API key field in the app's "Voice settings".
Google Cloud includes a limited free tier (around 1 million characters/month) for
Text-to-Speech; usage beyond that is billed to the linked Visa/Mastercard.