Window.onload() Set Element Value That Has Not Been Loaded?

I have a textarea on the page. When the page
loads, I need it to have some default text (which will be generated
dynamically)

so I did something like this

function init()
{
document.getElementById("TexareaID").value = "default text";
}

window.onload
{
init();
}

but the problem is firefox always return "TextAreaID" has no properties
because the textbox hasn't loaded it yet..it seems. How do I get around
that? (I know mabye i can insert the script below the textbox.. but
that's ugly. Any other ways?