I went with this solution for now. I'll edit my answer if it gets supported by the library. This is a feature in their roadmap for now (see this issue).
from multiprocessing import Process
from time import sleep
import win32con
import win32gui
from dearpygui.core import *
from dearpygui.simple import *
with window('MainWindow', width=500, height=500):
add_button('Read screen')
if __name__ == '__main__':
# you have to make a new process for this in order to be able
# to call win32gui.FindWindow on your displayed window
p = Process(target=start_dearpygui)
p.start()
# sleep for a while to let your window get displayed
sleep(4)
hwnd = win32gui.FindWindow('DearPyGui', None)
win32gui.SetWindowPos(hwnd, win32con.HWND_TOPMOST, 100, 100, 300, 200, 0)