Ho risolto così:

Grazie a tutti per l'aiuto!

codice:
  
<center>


<h1>Ricerca articoli</h1></p>  
<form name="form" action="#" method="GET" onsubmit="open_win()"> <input type="text" value="" size="15" name="codsam" id="codsam" /> 
 <input type="submit" value="Cerca"/>                              
</form> 
</center>  
<script> function open_win() { var URL = 'https://www.suppliesnow.nl/?lid=2&tc=search&search=' var cod = document.getElementById('codsam').value window.open(URL+cod,"_blank"); } 
</script>