I knwo there's no maximize option in taskbar.

What I ment was the old keybord shortcut to the actual window menus by pressing at and navigating with arrow keys. Atleast those for me those menus  (up on the top bar of the window, file, edit and such) were showing after alt and first arrow key press.
You just have to make sure poser main window is selected from taskbar even though it doesn't show.

right click on the taskbar "icon" doesn't have maximize option.