Here some text and images: 

First of all go to area where you want to add something 

Then when you are in, type this command in chat: /gml trace(GameCont.area) (for example our result is - oasis) 

Result which you get as answer is name of area 

Then, what do you want to add? 

If you want to attach an already existing spawn pool, go and find which one you need (oasis as for example) 

Now go to 743rd line and type this 

Some notes 

1st. Name of custom area should be in " " or else you will get error. 

2nd. Use same pool and weight pool, because there can be lesser weight numbers than mobs and you will get error when you enter the area for which you did this. 

You can also create your own spawn pool and use it for your area. 

So, go to 89th line Copy any spawn pool or type by yourself a new one 

Tooltip like this > "// Empty" isn't necessary

 Give name for spawn pool, and weight spawn pool as for example global.customarea_pool global.customarea_pool_w

 You can change only this "customarea" or else you can have some troubles 

And also try to don't use any numbers, special digits or anything else, just letters, because of possible errors. 

Everything else i tried to explain on this 2 more screenshot. 

Good luck in creating new spawn pools or just cool thing for custom areas and i hope i see you soon ;).