Thank you for posting your query on Microsoft Q&A, from above description I could understand that you are looking for way to access an API with access token issued to user account with Azure B2C.
Please do correct me if this is not the case by responding in the comments section.
Yes, you could Secure an Azure API Management API with Azure AD B2C and have it accessed with a B2C user account via an access token issued by B2C endpoint, PFB steps for the same
- Get Azure AD B2C application ID
- Get a token issuer endpoint
- Configure the inbound policy in Azure API Management
- Validate secure API access
- Support multiple applications and issuers
- Migrate to b2clogin.com
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