Yep! I’ve since figured it out.
The format is:
motherland x y
- Where x is the race:
Human
Undead
Orc
Nightelf
Nightelfex
Humanex
Undeadex
Orcex - And y is a number referencing the place in the campaign counting the interludes as well.
So for the Bonus Campaign:
Motherland orcex 1.
That should put you right at the start of the Bonus Campaign.