added a SendAsync method to IRestClient that takes an IEnumerable<KeyValuePair<string?, string?>> in order to be able to form a FormUrlEncodedContent which you need for getting the tokens.

src/Discord.Net.Core/Net/Rest/IRestClient.cs

@@ -33,6 +33,7 @@ namespace Discord.Net.Rest
         /// <returns></returns>
         Task<RestResponse> SendAsync(string method, string endpoint, CancellationToken cancelToken, bool headerOnly = false, string reason = null);
         Task<RestResponse> SendAsync(string method, string endpoint, string json, CancellationToken cancelToken, bool headerOnly = false, string reason = null);
+        Task<RestResponse> SendAsync(string method, string endpoint, IEnumerable<KeyValuePair<string?, string?>> formDataContent, CancellationToken cancelToken, bool headerOnly, string reason = null);
         Task<RestResponse> SendAsync(string method, string endpoint, IReadOnlyDictionary<string, object> multipartParams, CancellationToken cancelToken, bool headerOnly = false, string reason = null);
     }
 }