You'll need to use IMAP for that.
You can use CommonFolders class to get the Drafts folder name, or you can use string name if you know it (e.g. "[Gmail]/Drafts").
First we create a message:
MailBuilder builder = new MailBuilder();
builder.Subject = "subject";
builder.From.Add(new MailBox("alice@email.com", "Alice"));
builder.To.Add(new MailBox("bob@email.com", "Bob"));
builder.Text = "This is plain text email";
IMail email = builder.Create();
Then connect to IMAP, select draft folder and upload the message:
using (Imap imap = new Imap())
{
imap.Connect("imap.example.com"); // or ConnectSSL for SSL
imap.UseBestLogin("user", "password");
CommonFolders folders = new CommonFolders(imap.GetFolders());
imap.UploadMessage(folders.Draft, email);
// -or- imap.UploadMessage("[Gmail]/Drafts", email);
imap.Close();
}