How to select an item in a TreeView using Win32 API

I am trying to automate a sequence of user inputs to a compiled application in C# using Win32 API. I do not have any source code for the application I am trying to control and it is running while I am trying to control it. In my code, I have a single button that, when clicked, needs to make a sequence of 3 inputs to the application I am trying to control:

1. Select an item in a treeview
2. Click a button
3. Click another button

The way it works is the button in step 2 performs an action depending on the item selected in the treeview in step 1. I am able to get the button clicks to work just fine by simply sending a message, but I cannot figure out how to select the TreeView item I want. The TreeView is static, so the items and layout will never change. It has the following layout:

-itemsA
-itemsB
--itemB1
-itemsC

Where itemB1 is the item that needs to be selected in order for the button clicks in steps 2 and 3 to work. By default ItemsB is collapsed, so I probably need to expand it before I can select ItemB1? Here is my code. I really appreciate any help!!

//Find Window API
[DllImport("User32.dll")]
public static extern Int32 FindWindow(String lpClassName, String lpWindowName);

//Find WindowEx API
[DllImport("user32.dll", SetLastError = true)]
public static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, string className, string windowTitle);

//Send Message API
[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern int SendMessage(int hWnd, int msg, int wParam, IntPtr lParam);


private const int BN_CLICKED = 245;

//Method called by button click
public static void Start()
{
    int hwnd = 0;
    int prod = 0;
    IntPtr hwndChild = IntPtr.Zero;
    IntPtr treeChild = IntPtr.Zero;
    IntPtr prodChild = IntPtr.Zero;

    hwnd = FindWindow(null, "Application");
    if (hwnd > 0)
    {
        //Get Handle for TreeView, THIS IS WHERE I AM STUCK!