I'm trying to edit EmailOctopus plugin, this is part of their "form.php" code:

<?php
if (!defined('ABSPATH')) {
    die('No direct access.');
}

/**
 * Filter: emailoctopus_class_names
 *
 * Add more class names.
 *
 * @since 2.0.0
 *
 * @param array $main_class_names Array of class names
 * @param int   $form_id          The form ID
 */
$main_class_names = apply_filters('emailoctopus_class_names', $main_class_names, $form_id);
?>
<div class="<?php echo esc_attr(implode(' ', $main_class_names));?>" <?php echo $appearance === 'custom' ? sprintf('style="%s"', implode(' ', $main_class_styles)) : '';?>>
    <form method="post" action="/<?php echo esc_attr($list_id); ?>/members/external-add" class="emailoctopus-form">
        <div class="emailoctopus-form-textarea-hidden" aria-hidden="true">
            <textarea class="emailoctopus-form-textarea-hidden" name="message_consent_required" aria-hidden="true"><?php echo esc_textarea(wp_kses_post($message_consent_required)); ?></textarea>
            <textarea class="emailoctopus-form-textarea-hidden" name="message_missing_email" aria-hidden="true"><?php echo esc_textarea(wp_kses_post($message_missing_email)); ?></textarea>
            <textarea class="emailoctopus-form-textarea-hidden" name="message_invalid_email" aria-hidden="true"><?php echo esc_textarea(wp_kses_post($message_invalid_email)); ?></textarea>
            <textarea class="emailoctopus-form-textarea-hidden" name="message_bot" aria-hidden="true"><?php echo esc_textarea(wp_kses_post($message_bot)); ?></textarea>
            <textarea class="emailoctopus-form-textarea-hidden" name="message_success" aria-hidden="true"><?php echo esc_textarea(wp_kses_post($message_success)); ?></textarea>
        </div>
        <?php
        $show_title = isset($show_title) ? $show_title : true;
        $show_description = isset($show_description) ? $show_description : true;

        if ($show_title) {
            printf('<h2 class="emailoctopus-heading">%s</h2>', wp_kses_post($form_data['title']));
        }
        if ($show_description) {
            printf('<p>%s</p>', wp_kses_post($form_data['description']));
        }
        ?>
        <p class="emailoctopus-success-message"></p>
        <p class="emailoctopus-error-message"></p>
        <div class="emailoctopus-form-copy-wrapper">
            <input type="hidden" name="emailoctopus_form_id" value="<?php echo absint($form_id); ?>" />
            <input type="hidden" name="emailoctopus_list_id" value="<?php echo esc_attr($list_id); ?>" />
            <?php
            foreach ($custom_fields as $custom_field) {
                echo '<div class="emailoctopus-form-row">';
                echo sprintf(
                    '<label><span class="emailoctopus-label">%s %s</span><br /><input type="%s" name="%s" class="emailoctopus-custom-fields" tabindex="%d" /></label>',
                    esc_html($custom_field['label']),
                    $custom_field['tag'] === 'EmailAddress' ? '<span class="required">*</span>' : '',
                    esc_attr(strtolower($custom_field['type'])),
                    esc_attr($custom_field['tag']),
                    absint($tabindex)
                );
                echo '</div>';
                $tabindex += 1;
            }

            if (isset($show_consent) && $show_consent) {
                ?>
                <div class="emailoctopus-form-row">
                    <label>
                        <input type="checkbox" name="consent" class="emailoctopus-consent" tabindex="<?php echo absint($tabindex); ?>" />&nbsp;<?php echo wp_kses_post($consent_content); ?>
                    </label>
                </div>
                <?php
                $tabindex += 1;
            }
            ?>
            <div class="emailoctopus-form-row-hp" aria-hidden="true">
                <!-- Do not remove this field, otherwise you risk bot signups -->
                <input type="text" name="hp<?php echo esc_attr($list_id); ?>" tabindex="-1" autocomplete="nope">
            </div>
            <div class="emailoctopus-form-row-subscribe">
                <?php
                if ($redirect) {
                    echo sprintf('<input type="hidden" name="successRedirectUrl" class="emailoctopus-success-redirect-url" value="%s" />', esc_attr($redirect_url));
                }
                ?>
                <button type="submit" tabindex="<?php echo absint($tabindex); ?>" style="<?php echo $appearance === 'custom' ? sprintf('background: %s; color: %s;', esc_attr($button_color), esc_attr($button_text_color)) : ''; ?>"><?php echo esc_html($message_submit); ?></button>
            </div>
            <?php
            if (isset($show_branding) && $show_branding) {
                ?>
                <div class="emailoctopus-referral">
                    <?php echo sprintf(__('Powered by %sEmailOctopus%s', 'emailoctopus'), '<a href=";utm_medium=wordpress_plugin" target="_blank" rel="noopener">', '</a>'); ?>
                </div>
                <?php
            }
            ?>
        </div>
    </form>
</div>

Only thing I want do to is edit this HTML and add a placeholder="something" here:

'<label><span class="emailoctopus-label">%s %s</span><br /><input type="%s" name="%s" class="emailoctopus-custom-fields" tabindex="%d" /></label>',

I could do this manually but I don't want to edit the plugin directly but rather I want to do it in functions.php of my child WordPress theme, how can I do that there?

EDIT: My attempt:

<?php
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles', 'my_theme_add_placeholder' );
function my_theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}

function my_theme_scripts() {
    wp_enqueue_script( 'my-great-script', 'C:/MAMP/htdocs/site/wp-content/plugins/emailoctopus/templates' . '/form.php', array( 'jquery' ), '1.0.0', true );
    jQuery('class:emailoctopus-custom-fields').attr('placeholder','Some New Text');
}
add_action( 'wp_enqueue_scripts', 'my_theme_scripts' );

?>