OAuth Connector - Implicit Grant without Prompt

I am creating a custom connector to an OAuth service that uses implicit grants (access token granted immediately) and no prompt. The username and password are passed directly in the HTTP request body. All connector documentation I have seen uses a LoginUri. How can I set up OAuth with an implicit grant and no prompt?

Each API request to the service includes the access token and API key. Ideally I would like to store the access and refresh tokens in the credential object once authentication is done and re-use the access token until it exires, then renew it with the refresh token. The API key I would like to be a parameter in the M queries, since the API key is not used during the authentication or renewal process.