Putasync 400 Bad Request c# - google APi works

I want to sent a base64pdf, via http put. Unfortunatly I get the 400 bad request error.

I tried it with the google rest api and there it worked fine.

string finalURL = upURL + pdf.Id + "/signedpdf";
string json = "{ 'base64Pdf' : '" + pdf.Base64Pdf + "' }";
using (var client = new HttpClient())
{
     var response = await client.PutAsync(finalURL, new StringContent(json, Encoding.UTF8, "application/json"));
     if (response.IsSuccessStatusCode)
        return true;
}