creating a dieline - how to turn *specific* lines of a shape into dashed lines

Hi, 

I'm creating a dieline for a packaging design (a toblerone-shaped box), so I've used the rectangle tool and the line segment tool to do this. So some of the lines need to be dashed lines, to suggest where the diecut is to be folded.

However, I can't seem to change specific lines of my dieline into dashed lines, ie to change just one line of the rectangle into dashed lines.When I try doing so, it changes all the lines of the rectangle into dashed lines. 

Any advice? 

Thanks!