Arkive says:
...is there any way to set the editor so that Mix Mapping is on by default? I use ghost blocks pretty regularly and it would be convenient if I didn't have to toggle it on each time I opened up the editor.

TGYoshi says:
Put this script file in {mpuserdata}\Scripts\EditorPlugins\Trackmania\, then reboot the game, then enable the plugin. It'll auto-enable blockmixing every time you boot the editor. Create the folders that don't exist.

https://images.mania-exchange.com/mpscripts/AutoEnableMix.Script.txt

({mpuserdata} is by default My Documents\ManiaPlanet)