[Map] custom pokemon encounters on grass

[PixelParas]

Hey so I'm pretty new to rom-hacking so sorry for the not so great code in advance

So I'm making a rom-hack and have added all the 18 alola pokemon's but I don't know how to make them randomly encounter on grass as I can't find them in the dropdown of advance map grass encounters

For now, I was just using a script to spawn them when the player collides with the tile but I later realized doing this for all the different grass patches wouldn't be a Great Idea
So how can I have let's say sylveon encounter on the grass?

Heres the code I was using for now

#dynamic 0x800000
#org @start
random 0x1
compare lastresult 0x0
if 0x1 goto @encounter
end 

#org @encounter
random 0x2
compare lastresult 0x0
if 0x1 call @sylveon

compare lastresult 0x1
if 0x1 call @umbreon

#org @sylveon
wildbattle 0x1B8 0x64 0x0
return

#org @umbreon
wildbattle 0xC5 0x64 0x0
return