multiple headers to the GET request

Hi @Anjali Agarwal , Welcome to Microsoft Q&A,

In GET request, you should not add Content to the request, because GET request has no request body. In addition, the Uri parameter of HttpRequestMessage should be a relative path, not client.BaseAddress. You can set client.BaseAddress as the base URL and use the relative path as the path of HttpRequestMessage.

var parameters = new PayParam
{
    token = "12345"
};
var json = JsonSerializer.Serialize(parameters);

using (HttpClient client = new HttpClient())
{
    client.BaseAddress = new Uri("https://test/test1");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    client.DefaultRequestHeaders.Add("ApiKey", "ABC12345");
    client.DefaultRequestHeaders.Add("MerchantCode", "Demo1");
    client.DefaultRequestHeaders.Add("MerchantKey", "Demo2");
    HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, ""); 
    HttpResponseMessage response = await client.SendAsync(request);

    if (response.IsSuccessStatusCode)
    {
        var jsonString = await response.Content.ReadAsStringAsync();
        var data = JsonSerializer.Deserialize<PayParam>(jsonString);
        string token = data?.token.ToString();
    }
    else
    {
        Console.WriteLine("Request failed: " + response.StatusCode);
    }
}

Best Regards,

Jiale

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". 

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.