XMLHttpRequest cannot load Request header field dataType is not allowed by Access-Control-Allow-Headers in preflight response.

Error:

When POST to WebMethod - ASP.NET

XMLHttpRequest cannot load Request header field dataType is not allowed by Access-Control-Allow-Headers in preflight response.

Solution: 

Add to web.config:

    <httpProtocol>

      <customHeaders>

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

        <add name="Access-Control-Allow-Headers" value="Content-Type, accessToken, signedInUserId, x-requested-with" />

      </customHeaders>

    </httpProtocol>