If you've associated an email domain to the identity provider, you can use the domain property to start a sign-in flow.
In case you need to use a different way to start the authentication flow with an identity provider, you can use the provider_id property. For example:
Mapping specific user email addresses with an identity provider.
Using different hints to identity the identity provider to be used by the user, like a company-specific page, IP address or other tracking information.