With the proxy API, you can start workflows of the workflow capability within SAP Build Process Automation out of your ABAP Environment.
The overall starting point is the class CL_SWF_CPWF_API_FACTORY_A4C. Once you have the API object of interface IF_SWF_CPWF_API, see the ABAP documentation on supported actions.
You've executed the integration steps in your ABAP environment. See Integrating SAP Build Process Automation.