Disabled form inputs do not appear in the request

Elements with the disabled attribute are not submitted or you can say their values are not posted (see the second bullet point under Step 3 in the HTML 5 spec for building the form data set).

I.e.,

<input type="textbox" name="Percentage" value="100" disabled="disabled" /> 

FYI, per 17.12.1 in the HTML 4 spec:

1. Disabled controls do not receive focus.
2. Disabled controls are skipped in tabbing navigation.
3. Disabled controls cannot be successfully posted.

You can use readonly attribute in your case, by doing this you will be able to post your field's data.

I.e.,

<input type="textbox" name="Percentage" value="100" readonly="readonly" />

FYI, per 17.12.2 in the HTML 4 spec:

1. Read-only elements receive focus but cannot be modified by the user.
2. Read-only elements are included in tabbing navigation.
3. Read-only elements are successfully posted.

Using Jquery and sending the data with ajax, you can solve your problem:

<script>

$('#form_id').submit(function() {
    $("#input_disabled_id").prop('disabled', false);

    //Rest of code
    })
</script>

To post values from disabled inputs in addition to enabled inputs, you can simply re-enable all of the form's inputs as it is being submitted.

<form onsubmit="this.querySelectorAll('input').forEach(i => i.disabled = false)">
    <!-- Re-enable all input elements on submit so they are all posted, 
         even if currently disabled. -->

    <!-- form content with input elements -->
</form>

If you prefer jQuery:

<form onsubmit="$(this).find('input').prop('disabled', false)">
    <!-- Re-enable all input elements on submit so they are all posted, 
         even if currently disabled. -->

    <!-- form content with input elements -->
</form>

For ASP.NET MVC C# Razor, you add the submit handler like this:

using (Html.BeginForm("ActionName", "ControllerName", FormMethod.Post,
    // Re-enable all input elements on submit so they are all posted, even if currently disabled.
    new { onsubmit = "this.querySelectorAll('input').forEach(i => i.disabled = false)" } ))
{
    <!-- form content with input elements -->
}

If you absolutely have to have the field disabled and pass the data you could use a javascript to input the same data into a hidden field (or just set the hidden field too). This would allow you to have it disabled but still post the data even though you'd be posting to another page.

I'm updating this answer since is very useful. Just add readonly to the input.

So the form will be:

<form action="/Media/Add">
    <input type="hidden" name="Id" value="123" />
    <input type="textbox" name="Percentage" value="100" readonly/>
</form>