function HTTPEncode(const AStr: string): string;
 const
  NoConversion = ['A'..'Z', 'a'..'z', '*', '@', '.', '_', '-'];
 var
  Sp, Rp: PChar;
 begin
  SetLength(Result, Length(AStr) * 3);
  Sp := PChar(AStr);
  Rp := PChar(Result);
  while Sp^ <> #0 do
  begin
  if Sp^ in NoConversion then
  Rp^ := Sp^
  else if Sp^ = ' ' then
  Rp^ := '+'
  else
  begin
  FormatBuf(Rp^, 3, '%%%.2x', 6, [Ord(Sp^)]);
  Inc(Rp, 2);
  end;
  Inc(Rp);
  Inc(Sp);
  end;
  SetLength(Result, Rp - PChar(Result));
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
  Edit1.Text := HTTPEncode(Edit1.Text);
 end;