Change text in request body

[Сергей]

Hello!

I want to change AAA to BBB in request body

I create script

import System;
import System.Windows.Forms;
import Fiddler;

function OnBeforeRequest(oSession: Session) {
        oSession.utilReplaceInRequest("AAA", "BBB");
       
} 

But nothing changed. Maybe i need to enable scripting somewhere? Or i got an error in script? 

Thanks!

[EricLaw]

Where exactly did you enter that content? Did you click Rules > Customize Rules?