B2C error page

Custom-policy.xml

<ClaimType Id="errorMessage">
	<DisplayName>There was an error</DisplayName>
	<DataType>string</DataType>
	<UserHelpText>Add help text here</UserHelpText>
	<UserInputType>Paragraph</UserInputType>
</ClaimType>

<ClaimsTransformations>
	<ClaimsTransformation Id="CreateRegErrorMessage" TransformationMethod="CreateStringClaim">
		<InputParameters>
			<InputParameter Id="value" DataType="string" Value="Please contact support - code 123568" />
		</InputParameters>
		<OutputClaims>
			<OutputClaim ClaimTypeReferenceId="errorMessage" TransformationClaimType="createdClaim" />
		</OutputClaims>
	</ClaimsTransformation>
</ClaimsTransformations>

<ClaimsProvider>
	<DisplayName>Self Asserted</DisplayName>
	<TechnicalProfiles>
		<TechnicalProfile Id="SelfAsserted-Error">
			<DisplayName>Error message</DisplayName>
			<Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
			<Metadata>
				<Item Key="ContentDefinitionReferenceId">api.selfasserted</Item>
				<Item Key="setting.showContinueButton">false</Item>
				<Item Key="setting.showCancelButton">false</Item>
			</Metadata>
			<InputClaims>
				<InputClaim ClaimTypeReferenceId="errorMessage"/>
			</InputClaims>
			<OutputClaims>
				<OutputClaim ClaimTypeReferenceId="errorMessage"/>
			</OutputClaims>
		</TechnicalProfile>

		<TechnicalProfile Id="SelfAsserted-RegError">
			<InputClaimsTransformations>
				<InputClaimsTransformation ReferenceId="CreateRegErrorMessage" />
			</InputClaimsTransformations>
			<IncludeTechnicalProfile ReferenceId="SelfAsserted-Error" />
		</TechnicalProfile>

	</TechnicalProfiles>
</ClaimsProvider>

<OrchestrationStep Order="2" Type="ClaimsExchange">
	<Preconditions>
		<Precondition Type="ClaimEquals" ExecuteActionsIf="true">
			<Value>extension_Flag</Value>
			<Value>False</Value>
			<Action>SkipThisOrchestrationStep</Action>
		</Precondition>
	</Preconditions>
	<ClaimsExchanges>
		<ClaimsExchange Id="SelfAssertedRegError" TechnicalProfileReferenceId="SelfAsserted-RegError" />
	</ClaimsExchanges>
</OrchestrationStep>

https://medium.com/the-new-control-plane/creating-an-error-page-for-an-azure-ad-b2c-custom-policy-flow-fb2692a3b50f