your question is not clear. will the front-end pass credentials to the backend? this would typically be a jwt token.
scaffolding is for razor projects, so you would probably scaffold the backend. then add a controller action that return a jwt token for username / password. you would then create a blazor login page that calls the token api. repeat for any other identity pages you want.
you will also want to create custom blazor authentication provider, and set the user and save jwt token on the blazor login page. you may also want to supply a refresh token api or just require a login when the token expires.
if only the frontend will use authentication, then scaffold the UI. this will add razor pages for login, register, etc. your blazor app should redirect to razor login page when authentication is required. see sample blazor login control. you can also rewrite the login pages in blazor (again this will require a custom authentication provider). google for samples