If the B2B users sign in with their own accounts in their IDP, the prompt like the Microsoft Login page will have to show up. However
Azure Active Directory B2C (Azure AD B2C) would be a solution for this use case. Azure AD B2C is a customer identity access management solution that provides you the scalability, availability, and reliability you need. It allows you to customize and control how users sign up, sign in, and manage their profiles when using your applications.
You can customize the entire user interface (UI) so that Azure AD B2C looks and feels like it's a native part of your application. This means you can use your own login page without the Microsoft login pop-up.
Here's some relevant documentation and resources to help you get started:
- Azure AD B2C Documentation: https://docs.microsoft.com/en-us/azure/active-directory-b2c/
- Customize the user interface with HTML templates in Azure Active Directory B2C: https://docs.microsoft.com/en-us/azure/active-directory-b2c/customize-ui-with-html?pivots=b2c-user-flow
- Code samples for Azure AD B2C: https://github.com/azure-ad-b2c/samples
Please evaluate and let me know if this would work for you.