Sometimes, it just isn't a thing.
Some concepts from the domain aren't natural to model as objects. Forcing the required domain functionality to be the responsibility of an entities or value objects either distorts the definition of a model-based object or adds meaningless artificial objects.
When a significant process or transformation in the domain is not a natural responsibility of an entities or value objects, add an operation to the model as a standalone interface declared as a services. Define the interface in terms of the language of the model and make sure the operation name is part of the ubiquitous language. Make the services stateless.