Hi Bohan
The API is named IfmSetMatFlowRechargeValue. You can find below the documentation for both C++ and Python, respectively:
http://www.feflow.info/html/help75/feflow/13_Programming/IFM/API/IfmSetMatFlowRechargeValue.html
https://dhi.github.io/ifm/api_doc.html?highlight=recharge#ifm.FeflowDoc.setMatFlowRechargeValue
Best
Carlos