One solution is to use the Microsoft UI Automation technology. It's shipped out-of-the-box with Windows since Vista. It's usable from .NET but also from C++ using COM.

Here is a short C++ console application example that displays the class name of the UI Automation Element currently at the middle of the desktop window, each second (you can have it run and see what it displays):

int _tmain(int argc, _TCHAR* argv[])
{
    CoInitialize(NULL);
    IUIAutomation *pAutomation; // requires Uiautomation.h
    HRESULT hr = CoCreateInstance(__uuidof(CUIAutomation), NULL, CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (LPVOID *)&pAutomation);
    if (SUCCEEDED(hr))
    {
        RECT rc;
        GetWindowRect(GetDesktopWindow(), &rc);
        POINT center;
        center.x = (rc.right - rc.left) / 2;
        center.y = (rc.bottom - rc.top) / 2;
        printf("center x:%i y:%i'\n", center.x, center.y);
        do
        {
            IUIAutomationElement *pElement;
            hr = pAutomation->ElementFromPoint(center, &pElement);
            if (SUCCEEDED(hr))
            {
                BSTR str;
                hr = pElement->get_CurrentClassName(&str);
                if (SUCCEEDED(hr))
                {
                    printf("element name:'%S'\n", str);
                    ::SysFreeString(str);
                }
                pElement->Release();
            }
            Sleep(1000);
        }
        while(TRUE);
        pAutomation->Release();
    }

    CoUninitialize();
    return 0;
}

From this sample, what you can do is launch the application you want to automate and see if the sample detects it (it should).

You could also use the UISpy tool to display the full tree of what can be automated in your target app. You should see the windows and other elements (text field) of this target app and you should see the element displayed by the console application example.

From the pElement discovered in the sample, you can call FindFirst with the proper condition (class name, name, control type, etc...) to get to the text field. From this text field, you would use one of the UI Automation Patterns that should be available (probably TextPattern or ValuePattern) to get or set the text itself.

The cool thing is you can use the UISpy tool to check all this is possible before actually coding it.