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// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
    
     $fields['account']['account_username']['placeholder'] = '';
     $fields['account']['account_username']['label'] = 'Nome Utente del tuo Account';  
     
     $fields['account']['account_password']['placeholder'] = '';
     $fields['account']['account_password']['label'] = 'Password del tuo Account';
         
        
     return $fields;
}