Problem statement: How do I authenticate users through a specific Connection?
Solution: You can accomplish this by specifying a connection parameter in the authorize request of your authentication flow - It’s important to note that the parameter must be the name of the connection (i.e. google-oauth2, Username-Password-Authentication, etc.) as opposed to the identifier or ID. The configuration of this will depend on whether you are simply constructing the authorize request yourself, or using an Auth0 SDK.
Reference Materials 
- https://auth0.com/docs/authenticate/database-connections
- https://auth0.com/docs/authenticate/identity-providers
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