Javascript not working on form load

Hi folks,

I want to disable a new button + new in a look up for this I am writing a javascript to hide the + new button.
when i checked its HTML i found it is inside a div whoes id is XXX_productgrade and its attribute which is
highlighted in yellow colour is set to "1" when I changed it to "0" it was disabled .

So this i wanted to do  by javascript  and got error . I was calling this function on formLoad event 

function test (){

      var h1 = document.getElementById("angus_productgrade");   // here h1 is returning null
      h1.setAttribute("isinlinenewenabled", 0);

} 

Please help me  regarding this.