How to define unknown scalars for a stitched schema in HotChocolate

Question

I want to stitched two schemas via HotChocolate. One of them is provided by a graphql-dotnet implementation. In that one there is the following definition:

scalar DateOnly

This causes the following error in the stitched graphql schema:

HotChocolate.SchemaException: For more details look at the `Errors` property.

1. Unable to resolve type reference `DateOnly`. (HotChocolate.Types.ObjectType)
   at HotChocolate.Configuration.RegisteredType.GetType[T](TypeReference typeRef)

I tried to solve it by creating a custom Scalar

 public class DateOnlyType : ScalarType<DateOnly, StringValueNode>
 {
    public DateOnlyType() : base("DateOnly")
    {
    }
    ....

and registering it

builder.Services
    .AddGraphQLServer()
    .AddType(new DateOnlyType())
    .AddRemoteSchema("thefailingschema");

But that has no effect.

Same problem exists with new fusion package.

Any help appreciated. Thanks