Upload file using jquery

I'm using this code to upload file but I get an error:

"Value should match pattern '\\A(?:(/(.|[\\r\\n])*)|(ns:[0-9]+(/.*)?)|(id:.*))\\z' (Parameter 'path')"

var client = new DropboxClient("<access_token>, "<access key>", "<secret key>");
try
{
await Upload(client, "https://www.dropbox.com/sh/...dl=0/", <image filename>, "AvatarImage"); -- this url is a link to share folder
}
catch (Exception ex)
{
var error = ex.Message + ex.Source;
}

async Task Upload(DropboxClient dbx, string folder, string file, string content)
{
using (var mem = new MemoryStream(Encoding.UTF8.GetBytes(content)))
{
await dbx.Files.UploadAsync(
folder + "/" + file,
WriteMode.Overwrite.Instance,
body: mem);
}
}