alt plus left click and grab and drag where you wanna place the window on your desktop.
Then look through hidden fluxbox file for the keys file if you have a hot key to make the window always open there.
Code:
$ cd ~/.fluxbox
harry@chromebook:~/.fluxbox
$ ls
apps lastwallpaper menu-numix-bevel overlay startup
init menu menu-numix-square personal windowmenu
keys menu-applications menu-papirus slitlist
harry@chromebook:~/.fluxbox
Also, In Fluxbox, there is an option in the ~/fluxbox/init file to define the default location of new windows.
You can search the web on how to set yours up.