Generate username from firstname and last name PMPro.

Generate username.php

<?php
/**
 * This requires billing fields to be enabled at checkout.
 * Generate a username from firstname and lastname fields. If name exists will try to generate a random number after the username.
 * Add this code to your site by following this guide - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function my_generate_username_at_checkout() {
  
	//check for level as well to make sure we're on checkout page
	if(empty($_REQUEST['level'])) {
		return;
    }
    
    if ( ! empty( $_REQUEST['bfirstname'] ) && ! empty( $_REQUEST['blastname'] ) ) {
        $joined_name = $_REQUEST['bfirstname'] . $_REQUEST['blastname'];

        $username = my_generate_username( $joined_name );
    } else {
        $username = '';
    }
    
	  
	if ( ! empty( $_REQUEST['bemail'] ) ) {
		$_REQUEST['username'] = $username;
	}
    
	if ( ! empty( $_POST['bemail'] ) ) {
		$_POST['username'] = $username;
	}
    
	if( ! empty( $_GET['bemail'] ) ) {
		$_GET['username'] = $username;
	}
     
  }
add_action('init', 'my_generate_username_at_checkout');

/**
 * Generate a username from a user's email address.
 * Helper function.
 */
function my_generate_username( $name ) {

	while( username_exists( $name ) ) {
		$name .= random_int( 0, 9999 );
	}

    return sanitize_text_field( $name );
}