If Google has already created a key for you, copy it there. Otherwise, click Create API key and follow the prompts to select/create a Google Cloud project.
Copy the key and store it somewhere secure.
Note: Veo 3.1 has no free tier -- billing must be enabled on your Google Cloud project, or generation will fail.
Brands
Developer API
Subscribe to get an API key and call Forge's /api/v1/generate from your own backend, with the same brand-consistency options as the dashboard.