Multiple lines in a indicator, but how to select the second buffer?

Using iCustom(Symbol(),0,"indicator",0,1); how am I supposed to use the second or third buffer?
I have searched on the official mql website, but couldn't find it.

in your code the 0 refers to the 1st buffer :

here :
iCustom(Symbol(),0,"indicator",0,1);

so to use the 2nd buffer simple change it to 1

but remember inbetween the buffer and the indy name you must put in the external adjustable variables if the custom indy you are calling has any.