You type 'if (' without the closing bracket and it expands to the following.
if (|)
(The | is simply the blinking caret, or the place where you are typing.)
Then you type the expression and go to the end of the line by pressing <End>, or simply using a right-arrow key press or two.
if (point_in_rectangle(mouse_x,mouse_y,x,y,x+sprite_width,y+sprite_height|))
if (point_in_rectangle(mouse_x,mouse_y,x,y,x+sprite_width,y+sprite_height))|
Then when you type a space and an opening bracket (' {') you get this.
if (point_in_rectangle(mouse_x,mouse_y,x,y,x+sprite_width,y+sprite_height)) {|}
And you just press enter to get this:
if (point_in_rectangle(mouse_x,mouse_y,x,y,x+sprite_width,y+sprite_height)) {
|
}
...where the indentation is added automatically.
I think it would be possible to create this feature for the existing GM:Studio code editor by making a AutoHotKey script (it's probably made for this kind of jobs), I haven't used that program in a while though and it would probably be possible to use a Simulate Key Press DLL to make some kind of code helper utility in GML. :)