Use unique membership key as username with PMPro checkout.

my_init_unique_key_as_username.php

<?php // Do not copy this tag

// Use unique membership key as username with PMPro checkout. You'll also have to hide the username fields at checkout using CSS or a custom checkout page template.
// Paste the code below into a PMPro Customization Plugin: https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/

function salted_md5($encrypt_string) {
	// If no string is passed create one from timestamp
  if ( !$encrypt_string )
		$encrypt_string = rand(10000,99999);

	// Hash string using md5 encryption and salt it with current timestamp
  $encrypted_string = md5(AUTH_KEY . current_time('timestamp') . $encrypt_string . SECURE_AUTH_KEY);
	return $encrypted_string;
}

function my_init_unique_key_as_username()
{
	// Check that user isn't logged in before continuing
  if ( is_user_logged_in() )
		return;

	//check for level as well to make sure we're on checkout page
	if(empty($_REQUEST['level']))
		return;
	
	if(!empty($_REQUEST['bemail']))
		$_REQUEST['username'] = substr(salted_md5($_REQUEST['bemail']), 0, 10);
	
	if(!empty($_POST['bemail']))
		$_POST['username'] = substr(salted_md5($_POST['bemail']), 0, 10);
	
	if(!empty($_GET['bemail']))
		$_GET['username'] = substr(salted_md5($_POST['bemail']), 0, 10);
}

add_action('init', 'my_init_unique_key_as_username');

//Show it on the membership account page.
function pmprouku_pmpro_account_bullets_top()
{
	if(is_user_logged_in())
	{
		global $current_user;
		
		//get user login
		$user = get_user_by( 'id', $current_user->ID );
		$member_login = $user->user_login;
		
		//show it
		if(!empty($member_login))
		{
		?>
		<li><strong><?php _e("Username", "pmpro");?>:</strong> <?php echo $member_login?></li>
		<?php
		}
	}
}
add_action('pmpro_account_bullets_top', 'pmprouku_pmpro_account_bullets_top');
add_action('pmpro_invoice_bullets_top', 'pmprouku_pmpro_account_bullets_top');

//show unique username in confirmation emails
function pmprouku_pmpro_email_filter($email)
{
	global $wpdb;
 	
	//only update admin confirmation emails
	if(strpos($email->template, "checkout") !== false)
	{
		if(!empty($email->data) && !empty($email->data['user_login']))
		{
			$user = get_user_by("login", $email->data['user_login']);			
			if(!empty($user) && !empty($user->ID))
			{
				$member_login = $user->user_login;
				
				if(!empty($member_login))
					$email->body = str_replace("<p>Membership Level", "<p>Username: " . $member_login . "</p><p>Membership Level", $email->body);
			}
		}
	}
		
	return $email;
}
add_filter("pmpro_email_filter", "pmprouku_pmpro_email_filter", 30, 2);