API key authentication is not natively supported by Microsoft.AspNetCore.Identity, as it primarily handles cookie-based authentication. However, you can implement custom API key authentication by creating a custom authentication handler. This handler would validate API keys from the request header, associate them with users, and set up the HttpContext.User to continue processing the request as usual. You can store API keys in your database, link them to users, and then use the API key for authentication in API calls from tools like Excel or Power BI.