Add Custom Field and email message

I have a form in php in “single.php” from my site (wordpress). In it, the person adds your email address and a specific message is sent to it.
I would like to modify this form for a custom field is sent along the message she receives.
What I need to modify my code?

<? php
if (isset ($ _POST ['action']) && $ _POST ['action'] == 'email-att ") {
??? if (is_email ($ _POST ['email'])) {
???????? $ to = $ _POST ['email'];
???????? $ subject = "email subject line";
???????? $ message = "email message body";
???????? $ attachments = array ('https://full_url_to_file.zip');
???????? wp_mail ($ to, $ subject, $ message,'', $ attachments);
???????? echo 'Email sent! Check your email ';
???? else {}
???????? echo 'Please enter a valid email address';
???? }
}
?>

<form name="email-att" id="email-att" method="POST" action="">
Enter your emial: <br />
<input type="text" name="email" id="email"/> <br />
<input type="hidden" name="action" value="email-att"/> <br />
<input type="submit" name="submit" value="Submit" id="submit"/>
</ form>