The Urban Digital Twin (UDT) paradigm is increasingly adopted to monitor and simulate transport networks for smart city mobility planning and policy-making, as well as for analyzing the integration of innovative transportation technologies and services. Although several mature traffic simulators are available, their integration into broader UDT systems is still limited due to the lack of standardized APIs for scenario definition, management and execution. This paper presents a REST API designed to integrate the SUMO traffic simulator into UDT systems for smart cities, adopting a simulation-as-a-service approach. The proposed framework encapsulates SUMO and exposes its functionalities through an HTTP-based web API, enabling real-time simulation control, scenario modification, and data retrieval. This approach facilitates the development of UDTs which support planning, monitoring, and interactive what-if scenario analysis within microservice architectures. The paper presents integration guidelines of the proposed framework into UDT platforms for smart cities and early experimental evaluations for performance and scalability assessment.