Liste déroulante avec le premier champ vide

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<script language="JavaScript">  

var target = "blank" //Destination des URL: "blank", "top" "self" ou "nom du cadre" ex: #frame2 
function envoie(frm){  
URL = frm.mgros.options[frm.mgros.selectedIndex].value  
if (frm.mgros.selectedIndex != 0){  
if (target == "blank") mywindow = window.open(''+URL+'');  
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else parent.frames[target].location = URL;  
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</script>  




<form name="Form_1">  
<font color="#000099" size="12" face="Times New Roman"></font><select name="mgros" onchange="envoie(this.form)" style="font-family: Verdana; font-size: 10pt; background-color: teal; color: yellow;" size="1">  

<option value=""></option>  
<option value="http://www.repeatlab.com/Mangas">AB Mangas Replay</option>  
<option value="https://www.6play.fr/m6">M6 Replay </option>  
<option value="http://www.w9replay.fr">W9 Replay </option>  



</select>  
</form>