Updating custom header value added as DefaultRequestHeaders of HttpClient

The error I was getting: An item with the same key has already been added. Key: x

Example code for mahesh_ing answer:

var request = new HttpRequestMessage
{
    Method = this.method,
    RequestUri = new Uri(this.requestUri),
};

request.Headers.Add("Key", "Value");

var client = new System.Net.Http.HttpClient
{
    Timeout = this.timeout
};

return await client.SendAsync(request);

I added header to actual (current) request using HttpRequestMessage and replaced a call with SendAsync instead of GetAsync and it resolved my issue. Thanks @levent.