Go to the location your engine is installed to. My engine location is: “U:\Epic Games\UE_4.15”
In that folder navigate to the "Engine| and then the “Config” folders. Then find the file "Engine\Config" you should now be in something like "…\UE_4.15\Engine\Config"
You want to open/edit the file named “BaseEditorPerProjectUserSettings.ini”.
Find the section labeled “[/Script/EditorStyle.EditorStyleSettings]”
In that section you want to add a new line with the following:
AssetEditorOpenLocation=MainWindow
Save and close the file.
Now when you create a new project you will have open assets in the main window. If you want a different option … you’ll need to replace the “MainWindow” setting with whatever option you want to use.
As of Unreal Engine 5.1, [/Script/EditorStyle.EditorStyleSettings] has been changed to [/Script/UnrealEd.EditorStyleSettings].
There’s a less invasive way of doing this that works for all version of Unreal btw, according to this snippet you can put this into “My Documents/Unreal Engine/Engine/Config/UserEditorPerProjectUserSettings.ini” and it will make it so that AssetEditorOpenLocation is "MainWindow" for ALL UE Editor versions on that computer.