Where/How to initialize a Singleton class #8632

Where can I initialize a singleton class at application startup that has access to configuration (connection string) and Application Services? To give some background, my team is working to integrate a package from WorkflowEngine.io.
https://github.com/optimajet/WorkflowEngine.NET https://workflowengine.io/documentation/how-to-integrate/ Examples of the package integrated in ABP/Angular stack do not show how to integrate with an existing API. I am working on integrating the package using ABP DI and exisitng framework to initialize the Singleton instance of the WorkflowManager. It needs to be initialized somewhere that I have access to my application services and configuration, so that I can use DI to pass in the required services.