Adding an image watermark in C# with HttpClient

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

// You can get an API key at https://pdfshift.io
var apiKey = "sk_xxxxxxxxxxxx";

var client = new HttpClient();
client.DefaultRequestHeaders.Add("X-API-Key", apiKey);

var payload = new
{
    source = "https://www.example.com",
    // Add image watermark
    watermark = new
    {
        image = "https://example.com/watermark.png",
        opacity = 0.5,
        position = "center"
    }
};

var json = System.Text.Json.JsonSerializer.Serialize(payload);
var content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");

var response = await client.PostAsync("https://api.pdfshift.io/v3/convert/pdf", content);

// Handle errors:
if (response.StatusCode >= System.Net.HttpStatusCode.BadRequest)
{
    throw new Exception($"Request failed with status code {response.StatusCode}");
}

var result = await response.Content.ReadAsByteArrayAsync();

System.IO.File.WriteAllBytes("result.pdf", result);

Console.WriteLine("The PDF document was generated and saved to result.pdf");