How to add new javascript file

Hello,

We’ve been sent both the javascript and html to implement a new mortgage calculator (see attached file) for a client’s website but we’re struggling. So far, I’ve:

1. created the file mortgagecalc.js and saved this in udesign/scripts

2. added the following line of code to the header.php file in the theme (not sure if I should as we are using a child theme):

<script type="text/javascript" src="<?php bloginfo('template_url'); ?>/scripts/mortgagecalc.js"></script>

3. Added the HTML to the relevant page to display the calculator https://astonandco.co.uk/financial-services/

I’m sure I’ve probably missed something but don’t know what! Can anyone help?

Thanks in advance