This will work, create a hook file and add the following code.

-----------

<?php

use WHMCS\Database\Capsule;


function decrypt_ticket_password_custom_fields($vars) {

    $ticketID = Capsule::table('tbltickets')->where('tid', $_GET['tid'])->first()->id;

    $script = "<script type='text/javascript'>
        $(document).ready(function() {
    ";

    foreach ($vars['customfields'] as $field) {

        if ($field['type'] !== 'password') {
            continue;
        }

        $value = Capsule::table('tblcustomfieldsvalues')->where(['fieldid' => $field['id'], 'relid' => $ticketID])->first();

        if (count($value)) {
            $request = localAPI('DecryptPassword', ['password2' => $value->value]);

            $password = $request['password'];

            $script .= "
                $('div[menuitemname=\"{$field['name']}\"]').children('div').eq(1).html('{$password}');
            ";
        } else {
            $script .= "
                $('div[menuitemname=\"{$field['name']}\"]').children('div').eq(1).html('N/A');
            ";
        }



    }

    $script .= " });
        </script>
    ";
    $output['customfieldjavascript'] = $script;

    return $output;

}

add_hook('ClientAreaPageViewTicket', 1, 'decrypt_ticket_password_custom_fields');

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Then open your viewticket.tpl file in whatever theme you use and add the following variable at the bottom:

{$customfieldjavascript}

This will work for multiple custom fields that are of the type password. Other custom fields will remain untouched.