Getting custom claim from external identity provider

Hi all, i try to fetch additional custom claims from the oidc login with external identity provider. I see that the authorization request if fine (requesting the required scopes). I tried it with attribute-importer mapper for my identity provider, but failed. It is not some nested claim.
I am new to keycloak, so maybe i misunderstood some concepts.
Any help or helpful example would be great.
Thanks.