Hook page.create:before, how to edit user input?

I have not tested it but I think this should work. I also added the changeTitle method as this might be useful too here.

<?php

class CustomPage extends Page
{
    /**
     * Change the page title
     *
     * @param string $title
     * @param string|null $languageCode
     * @return static
     */
    public function changeTitle(string $title, string $languageCode = null)
    {
        $page = parent::changeTitle($title, $languageCode);

        return $page->changeSlug('my-new-slug', $languageCode);
    }

    /**
     * Creates and stores a new page
     *
     * @param array $props
     * @return static
     */
    public static function create(array $props)
    {
        $props['slug'] = 'my-new-slug';
        
        return parent::create($props);
    }

    /**
     * Updates the page data
     *
     * @param array|null $input
     * @param string|null $languageCode
     * @param bool $validate
     * @return static
     */
    public function update(array $input = null, string $languageCode = null, bool $validate = false)
    {
        $page = parent::update($input, $languageCode, $validate);

        return $page->changeSlug('my-new-slug', $languageCode);
    }
}