Multisite – create a category in specific site

What you need is switch_to_blog and maybe get_blog_id_from_url which you would use like this:

$blog_id = get_blog_id_from_url( 'www.example.com' );
switch_to_blog( $blog_id );
wp_create_category( 'mycategory', 0 );
restore_current_blog();