Dear I tried that but not working

It may be your call that is the problem

Have read of this http://www.html5rocks.com/en/tutorials/cors/#toc-types-of-cors-requests

Really appreciated for your fast reply , the shared link is useful but I will share my web config

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

    <remove name="Access-Control-Allow-Credentials" />

    <add name="Access-Control-Allow-Credentials" value="true" />
    <remove name="X-Frame-Options" />
    <add name="X-Frame-Options" value="sameorigin" />
    <remove name="X-Content-Type-Options" />
    <add name="X-Content-Type-Options" value="nosniff" />
    <remove name="Strict-Transport-Security" />
    <add name="Strict-Transport-Security" value="max-age=10886400" />
    <remove name="X-XSS-Protection" />
    <add name="X-XSS-Protection" value="1; mode=block" />
  </customHeaders>
</httpProtocol>

as well I tried adding custom class by registering enableCors public static class WebApiConfig { public static void Register(HttpConfiguration config) { config.EnableCors(new EnableCorsAttribute("*", "", ""));

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

Also added a data notation over my umbraco api controller [EnableCors("","","*",PreflightMaxAge =200)]

all of these couldn't help for sorry this issue getting me mad

check this article too.

https://piotrbach.com/umbracocms-enable-cors-requests-in-umbracoapicontroller

Really this worked on azure only by enabling cors origin of the app service itself , unfortunately we will pass azure for its expenses and move to smarter asp we deployed on it all get requests are working but remaining post request you can check it by submitting that form with any values open console tab finally thanks for your efforts.

http://testvue-001-site1.dtempurl.com/RatingForm

Although the thread is about azure the processes followed are not azure specific. Either your umbraco setup is incorrect or the way you are calling your API is incorrect, one or the other. We successfully use cors in with an umbracoapi and another site calling it.

Would like to mention that local environment working successfully with no problem

How are you calling your local setup from an external domain?

If it isn't working on your server then it is likely an iOS problem if you say it works locally.

My local I meant with it, My IIS server website calling it from another site

Sorry, I'm not understanding what you are trying to say.

What works and what doesn't?

If you have umbraco set up in a local iis website and can call it from another site then it isn't a problem with umbraco.

If you have it set up on another server in iis and that fails then it is a server issue.

For in details clarification MR/Reddick I have an app built with umbraco and act as umbraco API endpoint , on the other hand I have a vue app as frontend app let's head away vue app away it's not our matter . On building the backend app all APIs locally on postman and IIS website are working successfully even "calling it from another website on IIS" then after deployment on azure faced that mentioned issue of CORS preflight error but it's resolved by enabling CORS of azure app service in addition to applying https://piotrbach.com/umbracocms-enable-cors-requests-in-umbracoapicontroller . Cuz of highly expenses on azure we had to move on to any other hosting vendor after trying SmarterAsp that issue has been encountered again even if applying the previous URL steps. Eventually hope to find a reasonable cause for that.