@aarbuzik you can edit file /include/class.ticket.php, find this code ( i added whatsapp for new source).
static protected $sources = array(
'WhatsApp' => 'WhatsApp',
'Phone' =>
/* @trans / 'Phone',
'Email' =>
/ @trans */ 'Email',
'Web' =>
/* @trans */ 'Web',
'API' =>
/* @trans */ 'API',
'Other' =>
/* @trans */ 'Other',
);
then edit file include/staff/ticket-open.inc.php., find this script
<?php
$source = $info['source'] ?: 'WhatsApp'; // change whatsapp as default
$sources = Ticket::getSources();
unset($sources['Web'], $sources['API']);
foreach ($sources as $k => $v)
echo sprintf('<option value="%s" %s>%s</option>',
$k,
($source == $k ) ? 'selected="selected"' : '',
$v);
?>
</select>
then update ost_ticket table on database, find field 17 source. fill this
'Web','Whatsapp','Email','Phone','API','Other'
then save the config