setup custom header:

 <httpProtocol>
<customHeaders>
<add name="Access-Control-Allow-Origin" value="*" />
<add name="Access-Control-Allow-Headers" value="*" />
<add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />
</customHeaders>
</httpProtocol>

Server cài đặt: 

Install-Package Microsoft.AspNet.WebApi.Cors

cấu hình:  https://csharp-video-tutorials.blogspot.com/2016/09/cross-origin-resource-sharing-aspnet.html 

EnableCorsAttribute cors = new EnableCorsAttribute("*", "*", "*");

config.EnableCors();

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Gọi Ajax:

$.ajax({
type: "POST",
url: 'change-your-url',
cache: false,
contentType: "application/x-www-form-urlencoded; charset=utf-8",
dataType: "json",
success: function (res) {
console.log(res);
},
error: function (xhr, textStatus, errorThrown) {
console.log(xhr, textStatus, errorThrown);
}
});