I am using Laserweb 4,

I am using Laserweb 4, and the commands I wrote in GCODE START are sent without a line break and are not understood by my Drawbot. How can I insert line break in my GCODE START window?

I don’t understand your problem, with enter key it can be enough.

In the GCODE START window I wrote:
M3S0
$1=255
G21
G90
M3 S250

And in the generated gcode, they appear:
M3S0 $1=255G21G90M3 S250

Without line breaks, and this line is not understood by my Drawbot.

If I write your gcodes at the beginning, that’s what I get:
M3S0
$1=255
G21
G90
M3 S250

; Pass 0

; First Move
G0 X4.10 Y0.10
; Generated by LaserWeb (lw.raster-to-gcode.js)

Anyway, $1=255 is not nececsary, and M3 S0 or M3 S255.