Here is the definition of the AuthnStateSpec for the USER_ATTRIBUTES_REQUIRED state:

public final static AuthnStateSpec<UserAttributesRequired> USER_ATTRIBUTES_REQUIRED = new AuthnStateSpec.Builder<UserAttributesRequired>()
	.status("USER_ATTRIBUTES_REQUIRED")
	.description("The user's username and attributes are required.")
	.modelClass(UserAttributesRequired.class)
	.action(ActionSpec.SUBMIT_USER_ATTRIBUTES)
	.action(CommonActionSpec.CANCEL_AUTHENTICATION)
	.build();

Here is the specification for the submitUserAttributes action:

public final static AuthnActionSpec<SubmitUserAttributes> SUBMIT_USER_ATTRIBUTES = new AuthnActionSpec.Builder<SubmitUserAttributes>()
	.id("submitUserAttributes")
	.description("Submit the user's username and attributes.")
	.modelClass(SubmitUserAttributes.class)
	.error(CommonErrorSpec.VALIDATION_ERROR)
	.errorDetail(ErrorDetailSpec.INVALID_ATTRIBUTE_NAME)
	.build();