Contact form 7 wordpress post form to different domain gives access control allow origin error - cross-domain

Question

I am trying to send contact us form details to Insightly CRM with Web to Lead html code. I changed action url of the form with the following code in my functions.php:
add_filter('wpcf7_form_action_url', 'wpcf7_custom_form_action_url');
function wpcf7_custom_form_action_url($url) {
global $post;
$id_to_change = 1315;
if($post->ID === $id_to_change)
return 'https://xxxx.insight.ly/WebToLead/Create';
else
return $url;
}
Everything looks fine on inspector but i get the following error on submit:
XMLHttpRequest cannot load https://xxxx.insight.ly/WebToLead/Create. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://xxxx.com' is therefore not allowed access.
I tried adding this to my functions.php:
add_action( 'init', 'allow_origin' );
function allow_origin() {
header("Access-Control-Allow-Origin: *");
}
I tried adding this in theme.php:
header("Access-Control-Allow-Origin: *");
I tried adding this to scripts.js of contact form 7 plugin:
$.ajax({
url: url,
++headers: { 'Access-Control-Allow-Origin': '*' },
++crossDomain: true,
I tried adding this to .htaccess:
Header always set Access-Control-Allow-Origin "*"
Nothing works :(
My server has Varnish 4 and ConfigServer Security & Firewall but i disabled both and still get the same error.
Please help me :(

Answer 1

After my research I noticed that javascript was the problem and it was not possible to bypass it with the Access-Control-Allow-Origin in any way on my side.
I used curl in a hooked php script so it can send the details to the other domain.
So I added a hook in my functions.php which I am overriding the wpcf7_mail_sent function:
add_filter('wpcf7_mail_sent', 'wpcf7_custom_mail_sent');
function wpcf7_custom_mail_sent($contact_form) {
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"https://xxxx.insight.ly/WebToLead/Create");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,http_build_query($_REQUEST));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_exec($ch);
curl_close($ch);
}