Thank you, this helped!
Unfortunately, the GuidGenerator is a protected property, so I won't be able to set it manually. Getting the ItemManager through DI is an option, however, I don't want it to use the actual ItemRepository but a Substitute, so I can return a fake return value on the FindByItemNrAndVariantAsync(...) method.
My question here is: Is there a way to provide the domain service with an implementation of the GuidGenerator without using the IDomainService interface? Or alternatively, is there a way to get the ItemManager through dependency injection but with a mocked ItemRepository?