Your pirate has to be hired and at the outpost you want to launch your sub from. Then, instead of dragging your finger from that outpost to another outpost, drag it from your outpost to the enemy sub. More specific, the location it will be when your sub will meet its opponent. You can see that the path the enemy sub will take, is being traced on the screen, so follow that.
Then, your sub will be displayed with the pirate already added.
(Note that since the other sub is approaching Adria, it's possible that when your finger is close to Adria, your sub's destination will snap to that outpost. Just be very careful to target the sub.
All of this is assuming that you need to use the pirate which I'm neither confirming or denying.