Configuring IIS to allow CORS requests

The problem:  sending ajax requests from HTML5 application on local machine to my API that hosted under IIS on Google Cloud.

The facts: 
The code looks like:
        $.post({
            url: apiEndpoint + "Init",
            contentType: "application/json; charset=utf-8",
            data: data,
            success: function (response) {
                logResponse("API response [Init]", response.d);
            },
            error: function (xhr, status, error) {
                logResponse('Fail to call init API', data);
            }
        });

The error was looks like:

Originally, there was "crossDomain: true" parameter in ajax request but it doesn't help.

There were crossdomain.xml and clientaccesspolicy.xml on the server and they were helpless as well.

The way:
I went to internet for advice and saw many and many solutions for ajax - nothing doesn't help.

What I tried:

- add to ajax 

                headers: {

                    'Access-Control-Allow-Origin': '*'

                }

- add on server

    <httpProtocol>

      <customHeaders>

        <add name="Access-Control-Allow-Origin" value="*" />

      </customHeaders>

    </httpProtocol>

But after that change on the server, the error was looks like:

So I added

     <add name="Access-Control-Allow-Headers" value="Content-Type" />

to <system.webServer> <httpProtocol> <customHeaders> section.

The conclusion: No need to add nothing to ajax request - not headers, not even "crossDomain: true". Just add to web.config <system.webServer> section:
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Origin" value="*" />
        <add name="Access-Control-Allow-Headers" value="Content-Type" />
      </customHeaders>
    </httpProtocol>

That's it. End of story. Unfortunately - the long story :-\