[kh99]

The short answer is no, there is no existing variable. If you were submitting a form instead of putting it as part of the url then you could make a form field called 'title'.

But if you create a plugin using hook location private_newpm_start and this code:

if (!$vbulletin->GPC_exists['title'])
{
    $vbulletin->input->clean_gpc('r', 'title', TYPE_NOHTML);
    $pm['title'] =& $vbulletin->GPC['title'];
}

then you can use 'title' to set the title for a new pm (I tried it and the title appeared in the text box but I didn't do any other testing).