SMBX - HOW DO YOU MAKE A LEVEL THE DEFAULT WORLD?

Hello everybody! I'm new to SMBX (and RMN for that matter), and I had a question about world and levels and stuff.

I want to have a warp level, where the entrances to other levels are located, but I'm not sure about how to make it a default world. More specifically, how do I set it up so whenever a level ends, this warp level is where your character returns?

I tried making it the first level you go to, but I want an intro level for story, and I'm not sure how to tell the program that the intro level is not the default warp level or whatever.

Anyway, I appreciate any help guys, you all seem so friendly and knowledgeable! :3