custompage with a form submit to private.php for multiple recipients pm

[pigsy]

I've a custompage with a form submitted to private.php with do=newpm that doesn't seems to work anymore with vbulletin 3.8.
We're used to select from this page multiple recipients for the private message filling a sendmultiple[] array with users id, but this variable doesnt seems to be understood from private.php, how to do that with vbulletin 3.8 ?
thanks!

[Lynne]

If you need help with the coding, the best thing to do is to actually post your code. Please use the code/html/php tags when you post it. Thanks.

[pigsy]

this is a simplified version of HTML that was working before the update:
<form action="/private.php" method="get" >

<input type="checkbox" name="sendmultiple[]" class="sendmultiple" value="55527" />
<input type="checkbox" name="sendmultiple[]" class="sendmultiple" value="241586" />
<!-- MANY sendmultiple checkbox here! -->

<input type="hidden" name="do" id="mydo" value="newpm" />
<input type="hidden" name="s" value="" />
<input type="hidden" name="userlist" value="buddy" />
<input type="hidden" name="ttype" value="" />
<input type="hidden" name="securitytoken" value="1245834853-2216c1a0f058f13f2eff8e092b9c87ed280aebd6" />
<input type="submit" name="pm" value="Send PM" />

</form>

now sendmultiple doesnt seems to be recognized and the private.php form is not already filled with the selected users.