When launching bannerlord, there should be a menu allowing you to select what module you want to play. Click "Configure" and click "Enable Cheats".
To actually activate the cheatmenu, you have to be in-game. On the world map, press "CTRL + ~". For me the key is in the top-left of the keyboard somewhere, but for other keyboards you may have to do some searching. Once you've pressed "CTRL + ~", a command line should appear. type "cheatmenu" and enter.
It's been a while since I've manually activated it so if anything's incorrect, someone else can help.