Red-alert

   red-alert
   Causes Red Alert status in a mission. This function ends the current mission, and moves to the next mission
   in the campaign under red alert status. There should only be one branch from a mission that uses this expression.
   Takes no arguments.

Comments

Despite what the SEXP description says, red-alert has been tested with a simple branching campaign and it appears that you can branch off of a red-alert mission.

Regardless, red-alert missions often cause errors in retail and SCP FreeSpace, so avoid them if it all possible. This can be done using campaign-persistent variables to "remember" the status of various craft just before the player departs. While most of the bugs that caused these errors have been fixed in FSO version 3.6.10, there is still no absolute guarantee that this sexp will work each time, every time.

If you want to make a red-alert occur as the player jumps out, create an event that waits for the player to hit ALT+J. Chain to it an event that causes the red-alert to occur once the ship has reached jumpout velocity (40 m/s) to give the red-alert a few seconds to go into effect.