I don't know much about the implicit grant type but I don't think Moodle supports it. For example, in the documentation for setting up Moodle to authenticate with Microsoft service it says Ensure implicit grant flow is disabled.
I think your options are: 1) use an OAuth 2 issuer that supports the authorization code grant type, or 2) implement this in Moodle yourself, either in-house or engage someone to do this.