Add E-Signatures to .NET (C#)

[ApiController]
[Route("api/[controller]")]
public class ContractController : ControllerBase
{
    private readonly HttpClient _http;

    public ContractController(HttpClient http) => _http = http;

    [HttpPost("send")]
    public async Task<IActionResult> Send([FromBody] ContractRequest req)
    {
        _http.DefaultRequestHeaders.Authorization =
            new AuthenticationHeaderValue("Bearer",
                Environment.GetEnvironmentVariable("SIGNBEE_API_KEY"));

        var response = await _http.PostAsJsonAsync(
            "https://signb.ee/api/send", req);
        var result = await response.Content.ReadAsStringAsync();
        return Ok(result);
    }
}