Hello, i am trying to make a window go on a specific desktop when it starts. How can i do that?

ex: When Steam gets launched, its window automatically goes on desktop 8 everytime

You should be able to do that through Window Actions.
You can either use the window menu of the window itself: either right-click on the title bar of the window or hit Alt+F3, then under More Actions select Special Application Settings. Or you can go into System Settings -> Window Management -> Window Rules, hit New... and then use the Detect Window Properties to select the target window/application. (Using the window menu is easier as it pre-populates the Window matching tab for you).

In the application/window-specific settings dialog, under the Size & Position tab you can select what Desktop you want the window/application to stay in.