How can I make sub-missions on my scenario, like in the East Wind campaign?

[CaptainDawson 93]

How can I make sub-missions on my scenario, like in the East Wind campaign? Is this something that I would have to script, or can I use a trigger to load a side mission? I don't need it for purposes of a campaign, I only want one mission with sub-missions activated when the player selects or activates it.

 

Any help appreciated, thanks!