"We have three cup finals and finals require passion, desire, commitment and going toe-to-toe with your opponent," Clark told BBC Radio Leeds.
"My players have all of those things and will show all of those attributes in these matches."
Town go into the play-offs having lost their final game of the regular season at Exeter, and Clark knows his team must improve on that performance if they are to reach the Championship.
"That was a little smack across the chops to wake us up. If we'd won the game we'd have only moved up one place possibly.
"I think it will actually help us.
"It is a very tough tie - Millwall are a very tough team and I rate their manager very, very highly. So let's get it on.
"But if we are going to win the play-offs we know we are going to have to beat some very good teams, so let's just go for it."
BBC Radio Leeds will have full match commentary on all of Huddersfield Town's play-off matches on 92.4 FM and 774 AM.
Source: BBC Sport
Source: BBC Sport