How to deal with json string by Delphi WebBroker

Question

I want to post a json string from brower like this:

{"user_name":"lined%"}

But when I read the content from the action, it just raise a exception :Error decoding URL style (%XX) encoded string at position 6

procedure TWebModule1.WebModule1DefaultHandlerAction(Sender: TObject; Request: TWebRequest; Response: TWebResponse; var Handled: Boolean);
var
  RequestContent :string;
begin
  RequestContent := Request.Content;
end;

The percent sign % in the json string cause this exception.

My solution is make the request content to base64 format, I think it's not good.

Any advice would be greatly appreciated, Thanks.