As in a specific monster spawns?
In the step event of obj_battle, look for the line shown in the screenshot and change irandom(totalMonsters) to e_mon.slime, if you want a slime to spawn, for 
example.
Viewing post in RPG Battle System for GameMaker Studio: 2 comments
The easiest is probably using a single global variable that you decide how it's set.
Eg you could draw invisible rectangles in the room, and give them a number that correlates to the monster enumerator eg:
monster = e_mon.slime;
Then in the collision event, with the player, you set the global variable to the monster variable
eg
global.monster = monster