You can create custom templates in WordPress defining the template name at the top of the template. This is a basic template that should help you get started. You can save your template with a file name like tmp-mycustomname.php and put it in your theme. You probably should create a child theme where you can save this custom template and add additional CSS without losing it if the client updates the theme.
<?php
/*
* Template Name: Template name goes here
*/
?>
<?php get_header(); ?>
<div class="row">
<div class="col-md-12">
Your content can go here.
<script> alert ("my javascript is working"); </script>
</div>
</div>
<?php get_footer(); ?>
Once your template has been created go ahead to pages > add new. Then you should see something like this where you can find the custom template in the dropdown.
Helpful links:
https://developer.wordpress.org/themes/advanced-topics/child-themes/
https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/