Undo Unsubscribe Link

Can someone help me to know where and how to add the code below? “To the bottom of current theme Functions.hph file – I have no idea where that is or how to do that. Thank you

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7. Bonus! If you have a sharp eye, you probably thought: “Okay, but the old Unsubscribe page contained an Undo Unsubscribe link, how can I add it to this new Unsubscribe page?”
The old Unsubscribe page
Here’s how to do it: add the following code to the bottom of your current theme’s functions.php file (wp-content/themes/your-theme/).
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/**
* function to return an undo unsbscribe string for MailPoet newsletters
* you could place it in the functions.php of your theme
* @return string
*/
function mpoet_get_undo_unsubscribe(){
if(class_exists(‘WYSIJA’) && !empty($_REQUEST[‘wysija-key’])){

$undo_paramsurl = array(
‘wysija-page’=>1,
‘controller’=>’confirm’,
‘action’=>’undounsubscribe’,
‘wysija-key’=>$_REQUEST[‘wysija-key’]
);

$model_config = WYSIJA::get(‘config’,’model’);
$link_undo_unsubscribe = WYSIJA::get_permalink($model_config->getValue(‘confirmation_page’),$undo_paramsurl);

$undo_unsubscribe = str_replace(
array(‘[link]’,'[/link]’),
array(‘‘,’‘),
‘<p><b>’.__(‘You made a mistake? [link]Undo unsubscribe[/link].’,WYSIJA)).'</b><p>’;

return $undo_unsubscribe;
}
return ”;
}

add_shortcode(‘mailpoet_undo_unsubscribe’, ‘mpoet_get_undo_unsubscribe’);
Then add this shortcode to your newly created Unsubscribe page: [mailpoet_undo_unsubscribe]
New shortcode

Tip: If you are using a custom Page Template you can “echo” the Undo link directly into your .php template:
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echo mpoet_get_undo_unsubscribe();
Here’s what it’s going to look like now:
New unsubscribe page

https://www.ads-software.com/plugins/wysija-newsletters/