create a marker (player start)
create a door (or any other object like a circle, a phone box etc) in entities, furnitures on the left panel
create a « win zone » on it
right click on the win zone
modify the properties of the win zone on the left panel, in ai system, in use :
by adding the name of the map ("myprojectmap2" or whatever you like but without extention) Apply change (bottom left)
of course, you need to create and save « myprojectmap2 » (with extension) and create something distinctive.
you need to return to « myprojectmap1 » (with extension) (the one with the portal) and export as standalone application.
and it should work.