Let me see if I understand;
- You send out an email to parents
- Email contains link to book a help session
- You want the link that they click on (in the message body of the received email) to auto-populate the email field of the help session booking form
Is that right?
You could do it if you have control of the email sending script and the session booking form page.
You could first edit the email sending script so that the link to the session booking page/form contained in the message body, has their email address appended as a query string;
Then at the top of the form page, use PHP to GET the query string (the email) and auto-populate the form field for the email address;
<?php $email_from_link = isset($_GET['email']) ? $_GET['email'] : '';?>
And for the form field – populate the ‘value’ attribute;
<input type="text" name="email" value="<?php echo $email_from_link;?>">