Text box field in signin.html

Hi Experts - I am trying to add text box in signin.html and trying to capture in signon peoplcode.

Here is what i am trying to do.

1. Add a text column to ask user to enter their phone number in addition to username & password.

2.Capture the phone number in signon peoplecode and validate against DB.

I tried simply adding text box html like below and tried %request.getheader or %request.parameter. both seems to be not working and its returning null value.

<input type="text" id="phonenumber">

What is the best way to achieve this. Thanks!