Create a new text file, right click in windows > New Text Document. This creates a text file in which you can paste the Response you generate in Grin++, safe the file. Try to select that file from within Bittrex. If you do not see that file, or they do not accept. You might have to chance the file extention of the file, from “.txt.” to “.slatepack”. This is the format they expect, a text file, that has as file extention .slatepack and contains the Response text you copied from Grin++.
Let me know if that works for you.