Orthanc supports DICOM Modality Worklists (MWL) through its plugin SDK. In other words, Orthanc can act as a worklist server (C-Find SCP), but an user-defined plugin must be provided to answer a list of worklists given a MWL query.
The rationale for using plugins instead of providing a built-in MWL mechanism, is that the way worklists are generated is very specific to the clinical flows and to the manufacturer of your RIS system. Indeed, depending on the situation, worklists can be generated either from HL7 messages, from calls to a Web service (e.g. through FHIR), or from a direct access to some RIS database. It is thus up to the Orthanc user to provide the worklist generator that is best suited for her use.
A sample plugin is available in the source
distribution of Orthanc to serve the worklists stored in some folder
on the filesystem. This sample plugin mimics the behavior of the
wlmscpfs command-line tool from the DCMTK software.
For more complex or integrated workflows where you must implement a custom MWL plugin, please check the documentation of the part of the Orthanc plugin SDK that is related to the management of worklists.