IF you can reach the window from the outside-- you can open it far enough
so the metal bar goes all the way outside. Then fold the bar back against
the glass and use a prop stick to hold the window open.
This will work until a strong gust of wind moves the window at which
time it will slam shut anyway.
Good old duct tape and a stick would be more sturdy and less
prone to fall out of place.
Breezes tend to be less at night so this should work for now anyway.
Welcome and Happy Camping!