Warnock was charged over comments he made to referee Mark Halsey as well as ordering his players not to shake the match official's hand after the final whistle.
The United boss was incensed by Halsey's decision to show a red card to Luke Varney two minutes into the second half as well as his failure to award a penalty for a foul on Tom Lees.
Warnock, who is facing a two-match touchline ban if he is found guilty, admitted he had exchanged words with Halsey but denied he was in breach of FA regulations.
"There were a lot of things which disappointed me on Sunday," said Warnock.
"The right thing for me to do, which I did on Wednesday, was to reply in writing. I put forward one or two things that happened.
"I've written a letter because it's a strange one. Without me going into detail, I've ticked two boxes. I've denied the charge and accepted it. I accept what I said but I deny the charge - if that makes sense."