Can I return a union type on `outputFields`?

Is there a way that I can return a union type on `outputFields`? I want to do something like it:

```graphql
mutation userRegister($input: UserRegisterInput!) {
userRegister(input: $input) {
... on UserRegisterResultSuccess {
user {
id
}
}
... on Error {
message
}
}
}
```

I would like to know if there's a way to do it on `outputFields` of a mutation.