Changed values during Page_Load don't show up

Question

I have a Project Server 2013 deployment on top of SharePoint2013 environment, and I deployed a webpart that registers a JavaScript on the page, during Page_Load. In this JavaScript I change the value of one input box of a Project Details Page. However, after the page is completely rendered, the new value doesn't appear on the input box, and the input box is still enabled. Strange thing is that I have added a JavaScript alert to the point in which the value is changed, and I can see the alert window, so the script runs. And another strange thing is that if I use IE Developer Tools to debug this and I set a breakpoint in the script, and follow it line by line, then when the page is rendered the value is changed and the box is disabled. How can I change the value of the input box? And why does it work if I debug on Developer Tools, but doesn't work if I just let the script run normally? This is my C# code:

public partial class SpecialIDUserControl : UserControl
{
  protected void Page_Load(object sender, EventArgs e)
  {
    try
    {
      String fieldGUID = "THIS_A_FIELD_GUID"
      String specialId = "bla bla bla"
      String javaScript = @"SetSpecialID('" + fieldGUID + "','" + specialId + "');";
      ScriptManager.RegisterStartupScript(this, GetType(), "initializeVar", javaScript, true);
    }
    catch (Exception ex)
    {
      // stuff
    }
  }
}

And this is my JavaScript

function SetSpecialID(fieldId, specialId) {
    var $puidElement = $("[GUID='" + fieldId + "']");
    $puidElement.val(specialId);
    WPDPParts[0].IsDirty = true; //makes the page dirty, so the user gets a notification for saving
    }
    $puidElement.attr('disabled', 'disabled'); //disables input box, since this field should not be manually changed
}

Answer 1

Try this:

id_field = "field_name";

$('input[title="'+id_field '"]').attr("value",specialId);
$('input[title="'+id_field +'"]').attr("LTValue",fieldGUID);