In this post, Consultant Marius Rochon gives an implementation of OAuth2 Extension Grants (OBO) using Azure AD B2C.

Currently, Azure B2C does not support the extension grant. This sample uses a custom web service (B2BOBOWeb) to provide a token endpoint, which handles the Extension Grant requests and communicates with B2C to respond with a valid response (access token). It uses a specific B2C tenant configured with custom journeys to handle this communication.

Note: this sample code, not intended for production use.

Get the sample code here.