DepResolvers.cs

public class SMDependencyResolver : SMDependencyScope, IDependencyResolver {
    public SMDependencyResolver(IContainer container) : base(container) { }
    
    public IDependencyScope BeginScope() {
        var child = Container.GetNestedContainer();
        return new SMDependencyScope(child);
    }
}

public class SMDependencyScope : IDependencyScope {
    public SMDependencyScope(IContainer container) {
        Container = container;
    }
    public IContainer Container { get; private set; }
    
    public object GetService(Type serviceType) {
        return Container.GetInstance(serviceType);
    }
    
    public IEnumerable<object> GetServices(Type serviceType) {
        return Container.GetAllInstances(serviceType).Cast<object>();
    }
    
    public void Dispose() {
        Container.Dispose();
        Container = null;
    }
}

// Then in your web API config global:
config.DependencyResolver = new StructureMapDependencyResolver(IoC.Container);
// IoC.Container is a Lazy<Container> for me