wpmem_post_products

/**
 * You can have all posts in a category set to require a membership
 * by using the wpmem_post_products filter hook and WP's has_category().
 *
 * @see https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_post_products/
 * @see https://developer.wordpress.org/reference/functions/has_category/
 *
 * NOTE: This filter requires that you also have the category blocked (or at least
 * all posts in the category marked as blocked). The simplest way to do that is to
 * use a filter to make sure any post in the category is viewed as blocked.
 * @see https://rocketgeek.com/code-snippets/use-a-filter-to-block-a-category/
 */
add_filter( 'wpmem_post_products', function( $post_products, $post_id ) {
    
    // What is the category:
    $category = 'my-blocked-category-slug';
    
    // What is the required membership:
    $membership = 'my-membership';
    
    $has_category = has_category( $category, $post_id );
    if ( $has_category ) {
        return array( $membership );
    }
    return $post_products;
}, 10, 2  );