Geany requests that the window manager places the find dialog in the same position it was last time it was open. This is also saved and used when Geany is closed and restarted. So just move it where you want and done.
This relies on the window manager/Wayland compositor telling the truth when Geany asks where the dialog is placed, and placing it back there when requested. This is not the case for all window managers/compositors, but thats nothing Geany can change.