You can have javascript do that for you

window.onload = function () {

// do stuff here

}

If you need parameters from your wicket page in the javascript function you can override renderHead and add the function there:

@Override

public void renderHead(IHeaderResponse response)

{

    super.renderHead(response);

    String bar = something;

    response.render(JavaScriptHeaderItem.forScript(window.onload = function () {var foo=' + bar + '}));

    // or

    response.render(OnDomReadyHeaderItem.forScript(functionToCall( + bar + );) ;

}