Shortcode used to on the archive template to add a “Add New Category Post” that should capture the current category and automatically check it off in the editor sidebar.

functions.php

/**
 * 1. The FSE Button Component (Generates a clean URL parameter)
 */
function design_garden_fse_quick_add_button() {
    if ( ! is_category() ) {
        return '';
    }

    $current_cat = get_queried_object();
    $cat_id      = $current_cat->term_id;
    $cat_name    = $current_cat->name;

    // We change the URL key here to preset_cat
    $add_post_url = admin_url( 'post-new.php?preset_cat=' . $cat_id );

    $output  = '<div class="wp-block-button quick-add-wrapper">';
    $output .= '  <a href="' . esc_url( $add_post_url ) . '" class="wp-block-button__link quick-add-btn">';
    $output .= '    + Add New ' . esc_html( $cat_name ) . ' Post';
    $output .= '  </a>';
    $output .= '</div>';

    return $output;
}
add_shortcode( 'category_quick_add', 'design_garden_fse_quick_add_button' );


/**
 * 2. The Block Editor Interceptor (Forces Gutenberg to check the box)
 */
function design_garden_preselect_editor_category() {
    global $pagenow;

    // Only run this script when creating a BRAND NEW post
    if ( 'post-new.php' !== $pagenow || ! isset( $_GET['preset_cat'] ) ) {
        return;
    }

    $cat_id = intval( $_GET['preset_cat'] );
    if ( ! $cat_id ) {
        return;
    }
    ?>
    <script type="text/javascript">
        document.addEventListener('DOMContentLoaded', function() {
            // Safely watch the Gutenberg JavaScript Data Registry
            if ( window.wp && wp.data ) {
                const wpData = wp.data;

                // Wait for the block editor data store to cycle open
                const unsubscribe = wpData.subscribe(() => {
                    const editorSelector = wpData.select('core/editor');

                    if ( editorSelector ) {
                        const currentCategories = editorSelector.getEditedPostAttribute('categories');

                        // If the post loads with no categories checked, check our preset ID
                        if ( currentCategories && currentCategories.length === 0 ) {
                            wpData.dispatch('core/editor').editPost({ categories: [<?php echo $cat_id; ?>] });

                            // Immediately stop listening to save performance
                            unsubscribe();
                        }
                    }
                });
            }
        });
    </script>
    <?php
}
add_action( 'admin_footer', 'design_garden_preselect_editor_category' );

style.css

/* ==========================================================================
   Quick Add Button Layout
   ========================================================================== */

/* Container Spacing */
.quick-add-wrapper {
    margin-top: 1.5rem;
    margin-bottom: 2rem;
}

/* Button Cosmetics matching your global variables */
.wp-block-button__link.quick-add-btn {
    background-color: var(--wp--preset--color--background) !important; 
    color: var(--wp--preset--color--background) !important;
    border: 1px solid var(--wp--preset--color--primary) !important;
    padding: 0.6rem 1.4rem !important;
    border-radius: 8px !important;
    font-size: var(--wp--preset--font-size--small) !important;
    font-weight: 600 !important;
    text-decoration: none !important;
    display: inline-block;
    transition: all 0.2s ease-in-out;
}

/* Hover Adjustments */
.wp-block-button__link.quick-add-btn:hover {
    background-color: var(--wp--preset--color--background) !important;
    color: var(--wp--preset--color--primary) !important;
    transform: translateY(-1px);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}