[Script] How do I insert a choosing Starter special screen (like in RSE) into Firered?

[EmoMickey49]

I wanna make it so that when I click on this machine in Oak's lab, it brings up a separate screen with pokeballs where I can choose my starter

I've seen other hacks such as GS Chronicles do this: https://imgur.com/a/xOAARnE
And it's also a similar concept to that used at the beginning of RSE: https://imgur.com/a/ndopsqZ

So how could I enter a similar script into my game?

I know this requires creating a new special command then calling that special in my script, but I have absolutely no clue on how to do this.

 

[Anon822]

Here is a tutorial for adding new specials.
As for the starter selection screen code, the only implementation I know of is birch murder. I guess you could alternatively copy the code from emerald.