Add Category Function

category-function.php

function my_theme_custom_post_taxonomy() {
    register_taxonomy(
            'myname_cat', 'postname', array(
        'hierarchical' => true,
        'label' => 'Categories',
        'query_var' => true,
        'show_admin_column' => true,
        'rewrite' => array(
            'slug' => 'my-category',
            'with_front' => true
        )
            )
    );
}

add_action('init', 'my_theme_custom_post_taxonomy');