<?php 
 
add_action('wpcf7_init', 'custom_code_generator');
 
function custom_code_generator(){
    wpcf7_add_form_tag('coupon_code', 'custom_code_handler');
}
 
function custom_code_handler($tag){
    $input_code_name = 'your_input';
 
    $charsList = '1234567890abcdefghijklmnopqrstuvwxyz';
 
    $randomString = '';
 
    //change the value of i to meet your requirements
    for ($i=0; $i < 10; i++){
        $randomString .= $charsList[rand(0, strlen($charsList))];
    }
 
    $finalCode = strtoupper($randomString);
 
    //create html and return
 
    $html = '<input type="hidden" name="'.$input_code_name.' "value=" '.$finalCode . ' " />';
 
    return $html;
 
}