To clarify csk8er59's comments:
Assuming the following URL:
formpage.php?email=email&domain=$domain
On your formpage.php, you would do something like either of these options:
<?PHP
$email = $_GET['email'];
$domain = $_GET['domain'];
echo "<input type=\"text\" name=\"email\" value=\"".$email."\">";
echo "<input type=\"text\" name=\"domain\" value=\"".$domain."\">";
?>
OR
<?PHP
$email = $_GET['email'];
$domain = $_GET['domain'];
?>
<input type="text" name="email" value=<?PHP echo $email; ?>">
<input type="text" name="email" value=<?PHP echo $domain; ?>">