How would I launch a program in a smaller window, rather than use GetDesktopWindow?

Currently I am launching a program that takes up the whole window and I have to wait for it to finish its test to get access to the rest of the screen. Below is the code that implements this:

    CWnd * pWnd = GetDesktopWindow(); //Where the screen gets taken up

    CRect rect;

    pWnd->GetClientRect(&rect);
    
    MoveWindow(&rect, TRUE);
    InvalidateRect(&rect, TRUE);
    UpdateWindow();

CreateWindow or CreateWindowEx seems to be useful but wanted to ask generally if anyone had any further input. Thank you!