Я думаю, что следующий код не требует пояснений.

#include 

static HWND textBoxInput;
static HWND button;
static HWND textBoxOutput;

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int CALLBACK WinMain(HINSTANCE hInstance,HINSTANCE hPrevInstance,LPSTR cmdLine,int nCmdShow)
{
    HWND hMainWindow;
    WNDCLASS wc = {};
    wc.lpfnWndProc = WindowProc;
    wc.lpszClassName = "Main's window class";
    wc.hInstance = hInstance;
    RegisterClass(&wc);


    hMainWindow = CreateWindow(wc.lpszClassName,"Append text main window",WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,CW_USEDEFAULT,500,400,NULL,NULL,hInstance,NULL);

    error=GetLastError();

    if(hMainWindow == NULL) return 1;

    textBoxInput = CreateWindowEx(WS_EX_CLIENTEDGE, "Edit", NULL,WS_CHILD | WS_VISIBLE | ES_AUTOHSCROLL, 10, 10, 300, 21, hMainWindow, NULL, NULL, NULL);

    button = CreateWindowEx(WS_EX_CLIENTEDGE,"Button","Append",WS_CHILD | WS_VISIBLE | ES_CENTER, 10, 41,75,30,hMainWindow,NULL,NULL,NULL); 

    textBoxOutput = CreateWindowEx(WS_EX_CLIENTEDGE,"Edit",TEXT("->This content is untouchable and unreadable!