In the scope of the Horizon 2020 InterFlex project, a lack of proper interoperable and open solutions in the state-of-the-art interfaces for flexibility activation on distribution level have been identified. Therefore, an API has been defined to ensure interoperability between system operators (mostly DSOs) and flexibility providers (e.g. aggregators and/or prosumers) together with a bidding process allowing to negotiate and activate flexibilities. Project Deliverable D3.4 provides a formal definition of the API and the proposed bidding process. Furthermore, an open-source reference implementation of the API has been developed on a FIWARE based cloud platform. This design aims at simplifying the integration of further external services (e.g. demand forecast) into this platform. A reference implementation is now available here.