HttpContext.Current is null in my web service

If you want to use HttpContext because the code has already been written as so; you need to add this to your web.config where your service resides:

<configuration>
    <system.serviceModel>
        <serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
    </system.serviceModel>
</configuration>

Correct else use below to read header

 var headers = OperationContext.Current.IncomingMessageProperties["httpRequest"];
                var apiToken = ((HttpRequestMessageProperty)headers).Headers["apiKey"];

From one of Microsoft's pages on the subject.

HttpContext: Current is always null when accessed from within a WCF service. Use RequestContext instead.