How to end custom story as a Demo?

[Zypherzemus]

I'd add you on steam, but I don't have enough games yet.

Here is what I want to happen.
When you first start off the hub level, all level doors are locked except one, you enter that level door, then come out and open another, then you get the key to a swing door in the hub level which has the key to the last level door (end of demo) All I want to happen is when you unlock the door, the screen fades to black and then the credits start.