Hi Rajan,
It's Look likes there are 2 different element id that you want to change.
1. Form 3 --> element_61
2. Form 5 --> element_23
To do this, you need edit "includes/view-functions.php". Try to go around line 40 ~ 45, you'll see these code :
$element_markup = <<<EOT
<li id="li_{$element->id}" {$error_class}>
<label class="description" for="element_{$element->id}">{$element->title} {$span_required}</label>
<div>
<input id="element_{$element->id}" name="element_{$element->id}" class="element text {$element->size}" type="text" value="{$element->default_value}" />
</div>{$guidelines} {$error_message}
</li>
EOT;
try to replace with this one
if (($_GET['id'] == 3 && $element->id == 61 ) || ($_GET['id'] == 5 || $element->id == 23)) {
$element_input = '<input id="thankyou" name="thankyou" class="element text '.$element->size.'" type="text" value="'.$element->default_value.'" />';
} else {
$element_input = '<input id="element_'.$element->id.'" name="element_'.$element->id.'" class="element text '.$element->size.'" type="text" value="'.$element->default_value.'" />';
}
$element_markup = <<<EOT
<li id="li_{$element->id}" {$error_class}>
<label class="description" for="element_{$element->id}">{$element->title} {$span_required}</label>
<div>
{$element_input}
</div>{$guidelines} {$error_message}
</li>
EOT;
that code will change element name properties to "thank you".