Craft 3 Plugin not returning custom field value
Found my answer here

// get $_POST['foo']['bar'], if it exists
$bar = Craft::$app-&gt;request-&gt;getBodyParam('foo.bar');


or inserting it into my code

Event::on(Elements::class, Elements::EVENT_AFTER_SAVE_ELEMENT, function(Event $event) 
{
    $request = Craft::$app-&gt;getRequest();

    if($request-&gt;getBodyParam('fields.sendEmail') == 1)
    {
        $html = $request-&gt;getBodyParam('fields.sendEmail');
        $subject = "testing";
        $mail = "example@gmail.com";
        $this-&gt;sendMail($html, $subject, $mail);
    }
});