In the section ID Token, create a plugin in the ID Token ACR Claim (Flow) property.
Use a Flow Selection Based OIDC ID Token ACR Value plugin if the requested acr_values should be returned in the ID token.
Use a Flow Condition Based OIDC ID Token ACR Value plugin if the returned acr_value should be derived from flow tags, i.e. if the should contain information about the exact authentication method used.