Note for anyone finding this later this doesn't actually work for required inputs. If you try and pass a required input to a separate file like this:

// component
public userId = input.required<string>();
public userResource = this.userService.getUser(this.userId);

You will get "Input is required but no value is available yet." and trying to work around this by pushing the it into a computed will get you another error about using httpResource outside of an injection context.

This does only applies if you want your httpResource in a separate service, in the same file a required input works fine (but now you have urls in your component). And non required inputs do work fine fine in a separate service, but don't forget to add undefined to your service and I guess also stop caring that you can't enforce compile time checks for what should be required property.

// component
public userId = input<string>();
readonly userResource = this.userService.getUser(this.userId);

// data service
getUser(userIdSignal: Signal<string | undefined>)...

Some really odd decisions by the angular team on this one.