UnionTypes in HotChocolate

Question

I am trying to copy this GraphQL schema (don't have access to its code):

type User {
  id: ID!
  email: String!
  role: UserRole!
}

type ForbiddenError {
  message: String!
}

type Mutation {
  login(input: LoginInput!): LoginOutput!
}

union LoginOutput = User | ForbiddenError

The frontend makes this request (and it cannot be modified):

mutation login{
  login(input:{usernameOrEmail:"user@example.com",password:"qwerty123"})
{
  ...on User{__typename id email role }
  ...on ForbiddenError{__typename message}
  }
}

I tried to create an interface that two objects, User and ForbiddenError, would share,

[UnionType("LoginOutput")]
public interface ILoginOutput
{
}

public class ForbiddenError : ILoginOutput
{
    public string Message { get; set; }
}

public class User : ILoginOutput
{
    public string Id { get; set; }
    
    public string Email { get; set; }

    public string Role { get; set; }
}

public ILoginOutput Login(
    string usernameOrEmail,
    string password,
    [Service] IHttpContextAccessor httpContextAccessor)
{
    <...>
    if (...)
    {
        return new ForbiddenError();
    }

    <...>
    return user;
}

but in this case, I end up with the following schema: And error in request:

Fragment cannot be spread here as objects of type "LoginPayload" can never be of type "User".

I tried to solve the problem by using an exception,

public class ForbiddenError : Exception
{
}

[Error<ForbiddenError>]
public User Login(
    string usernameOrEmail,
    string password,
    [Service] IHttpContextAccessor httpContextAccessor)
{
     <...>
}

but I ended up with the following schema And same error in request:

Fragment cannot be spread here as objects of type "LoginPayload" can never be of type "User".

How can I get the schema like the one in the first image? So that the specified query to GraphQL can be executed.