Manipulate a webpage to include a watermark - autohotkey

Question

I work with a web application that works only on Mozilla Firefox.
A particular task generates a page to print a letter to be mailed.
After printed the letter, my colleagues should stamp the brand of post office and manually write the send date on which the letter is sent.
Is there any way to manipulate the display of the page to include a line in HTML with the stamp image before printing?
Any ideas?
Thank you very much.

Answer 1

Is there any reason it needs to be done via autohotkey? If you have access to the code, you could just add a print stylesheet that overlays the stamp image.
SmashingMagazine has a fantastic guide to print stylesheets:
In essence, all you have to do is add this to your existing stylesheet:
#media print {
//css to add the stamp image overlay here
}

Answer 2

Thanks for feedback.
I figured it out.
That's my code:
#NoEnv
#Persistent
#SingleInstance force
SendMode Input
SetWorkingDir %A_ScriptDir%
Gui, Add, Button, gCorreios, Correios
Gui, Show
Return
Correios:
Sleep 100
Send, {ALT DOWN}{ESC}{ALT UP}
Send, {CTRLDOWN}f{CTRLUP}
Sleep 10
Send, Postado{ESC}
Sleep 10
Send, {SHIFTDOWN}{END}{SHIFTUP}{CTRLDOWN}c{CTRLUP}
ClipWait
Sleep 10
DataDaPostagem=%Clipboard%
ClipBoard =
Sleep 10
StringReplace, DataDaPostagem, DataDaPostagem, postado,, All
StringReplace, DataDaPostagem, DataDaPostagem, em,, All
StringReplace, DataDaPostagem, DataDaPostagem, :,, All
StringReplace, DataDaPostagem, DataDaPostagem, `,,, All
StringReplace, DataDaPostagem, DataDaPostagem, %A_SPACE%,, All
Sleep 10
Send, {SHIFT DOWN}{F4}{SHIFT UP}
WinWait, Scratchpad,
IfWinNotActive, Scratchpad, , WinActivate, Scratchpad,
WinWaitActive, Scratchpad,
Send, {CTRLDOWN}a{CTRLUP}{DEL}
Send, var correios=document.createElement("div");{ENTER}
Send, correios.id="correios";{ENTER}
Send, correios.style="background:url(http://i62.tinypic.com/169hxls.png) no-repeat; display:block; height:37px; padding:135px 0 0 0; position:absolute; right:0; text-align:center; top:0; width:171px; z-index:-1";{ENTER}
Send, correios.innerHTML="
Send, %DataDaPostagem%
Send, ";{ENTER}
Send, document.getElementsByTagName("body")[0].appendChild(correios);{ENTER}
Send, {CTRLDOWN}r{CTRLUP}
Sleep 10
Send, {CTRLDOWN}w{CTRLUP}
WinWait, Alterações não salvas,
IfWinNotActive, Alterações não salvas, , WinActivate, Alterações não salvas,
WinWaitActive, Alterações não salvas,
Send, d
Sleep 10
return
~^s::
IfWinActive, %A_ScriptName%
{
SplashTextOn,,,Updated script,
Sleep, 200
SplashTextOff
Reload
}
return