What is the rest api for authenticating keycloak provider's user from keycloak broker

I have two keycloak setups. One is keycloak broker and second is keycloak provider. The brokering works when we authenticate the provider’s user through the oidc idp configured as shown below:

Now i want to login using this user through some rest-api.
Is there any readymade restapi call available for the above operation or can somebody suggest any other way to perform this ?

So far, one Keycloak instance (KC 1) is brokering the other one, let’s say KC 2. This means that KC 1 is integrated with KC 2 using OpenID Connect with the Authorization Code Flow [1]. As a result, there is NO way to log in directly to KC 2 via a REST API through the KC 1 OIDC endpoints, which means that you MUST use a browser with the standard flow.

[1] RFC 6749 - The OAuth 2.0 Authorization Framework