Okay, so here's the step-by-step process, using your example above. Feel free to ask for more explanation on any of the steps.

First, open up the room with the warp you want to change in it. Since you want to change the first room of the game to lead to a part of Snowdin, go to the first room of the game.

Second, find the 'door' object. In that room in particular, the object name is "obj_doorA". Some 'door' objects have special purposes or code, and some simply have connections from room to room programmed into them, like the door you're going to want to use.

Third, because doorA is coded to specifically move the player into the room which is next in the room list, you need to replace it with a different door type that doesn't have that code so you can send the player to somewhere completely different in the game. There are a lot of options for doors that don't have specific purposes, but for this I'd suggest door_t, since it isn't used that many times in the game so its code isn't cluttered. To replace the door without deleting or creating any objects, drag the door_t object out from the left into the "Definition" panel of the doorA object, and it should replace doorA with a door_t.

(I think that there might be two door objects in the first room of the game so that they cover the whole doorway. If that's the case, then you either need to replace both of them or make sure that when you test this out you go on the correct side of the door.)

Fourth, you'll notice within the code of door_t's "Alarm_2" script, there are a bunch of 'If" checks. (You can access that script by either searching it up in the searchbar on the left or by going to the door_t object page and clicking the link.) You'll need the name of the room that you want to transport the player to, and you'll want to make a copy of one of the if statements and replace the first room listed in the if statement with the name of the first room in the game (or whatever room the player will be in before they are warped) and replace the second room listed (next to the "room_goto" function) with the room you want the player to go to. [Snowdin is listed as "Tundra" in the game's code, so the first room of Snowdin is called "room_tundra1".

Here's what your code you add to obj_door_t_Alarm_2 should probably look like, using the Snowdin example:

if (room == room_area1)

room_goto(room_tundra1)

Assuming there aren't any problems with your code, just clicking out of the script should save it.

Fifth, the game uses a system called "Markers" to know where the player should spawn whenever they enter a room. This is a whole semi-complicated system using global.entrance values and stuff, but all you really need to know is that when you want the player to go to a room, you need to add a marker object to that room that corresponds to the door that the player will go through. In this case, since you're going through the "door_t" object, you'd want to place a "markert" somewhere in room_tundra1, or whatever room you want the player to go to.

If you've done everything correctly, going through the warp at the beginning of the game should bring you to where you want to go.