Call DLL C++ functions form C# without block the form controls

I have an interesting study case for you :)

I have a problema with a wrapper integration. I made a Dll in write in C++. CLR Windows. This Dll is called form a C# application (for a the), till here all rigth.

The C# aplications is form a thrid part and I Cannot modify this source code.

When I call some C++ function since a button for example. The applications si bloqued an I can't doing any more until the C++ function return. I need that when I wating for a C++ function the different the formulary controlls has enabled again so on, I need some additional process.

I try to do it with async methods and treads but I can't figure out the way to deploy it.

C++ function

__declspec(dllexport) HANDLE openport(char *ComPort, int BR);

C# function:

[DllImport("mydll.dll")] public static extern IntPtr openport(string ComPort, int BR);

Thanks in advance for your help.

Regards.