I figured it out myself I needed $wpdb and a , after the db_name Here is the result.
$user_id = $wpdb->INSERT(
'wp_fisker',
array (
'fisker_fornavn' => apply_filters('pre_user_first_name', $fisker_fornavn),
'fisker_efternavn' => apply_filters('pre_user_last_name', $fisker_efternavn),
'password' => apply_filters('pre_user_user_pass', $password),
'telefon' => apply_filters('pre_user_telefon', $telefon),
'zip' => apply_filters('pre_user_zip', $zip),
'by_navn' => apply_filters('pre_user_by_navn', $by_navn),
'email' => apply_filters('pre_user_user_email', $email)
)
);
if( is_wp_error($user_id) ) {
$err="Error on user creation.";
} else {
do_action('user_register', $user_id);
$success="You\"re successfully register';
}