@dd2a4d73-6c29-4e39-95c8-ca50253941d8, Yes you can enable SSO for your existing application by registering that application with Azure B2C platform. The way you would be able to enable authentication using Azure B2C is by creating an Azure B2C policies (signup/signin policy). You can read more on this here.
Adding this policy, you would be able to use your own authentication forms and also get to use Azure AD B2C too. You can refer to the following sample for more understanding.
https://github.com/Azure-Samples/active-directory-b2c-dotnet-webapp-and-webapi
Hope this helps.
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