As per title. I can't find the option to do this in either the launcher nor the in-game menu.

You can select it before you start a scenario in the campaign (button "Settings" on the left). Note that the default is Simultaneous, but you can choose Classic turns in the drop down menu. See attached picture.

Thank you!

I wish they just had one menu for everything, not the launch menu, in-game menu, and campaign screen menu, he.