Leyton Orient, who finished the regular season in third place, 12 points ahead of Posh, were the better side at London Road and created the clearer goal scoring opportunities.
The O's could count themselves unfortunate to fall behind after 16 minutes as Britt Assombalonga's 33rd goal of the campaign should have been ruled out for offside.
However, Moses Odubajo levelled things up midway through the second half to leave the tie in the balance ahead of the second leg in east London on Tuesday night.
Ferguson said: "When we went in front I thought it was a really good opportunity for us because we always had the threat of the second goal, although I felt we lacked a bit of composure.
"But I think you've got to give Leyton Orient a lot of credit because they were the better side on the day and I think if I was Russell Slade I'd be a little bit disappointed with a draw because they had the better chances.
"We are in a great position going into the second game. We have a one-off game to get to Wembley again.
"I've said it before, in a one-off game I would be confident against any team. I think it will suit us more at their place, which I know sounds funny, but we played really well when we beat them there in the league season.
"But that will be another tight game. It may go to extra time, but we will be ready for it."
Peterborough went ahead when Danny Swanson's shot from just outside the area was parried by Jamie Jones into the path of Mark Little, who fired the ball back across the face of goal for Assombalonga to head home from close range.
Orient appealed for offside but no flag was forthcoming from the assistant referee, who was brilliantly positioned. Television replays confirmed Assombalonga was just offside.
The equaliser came when former Posh striker Shaun Batt won a header against two defenders inside the box to give David Mooney the chance to shoot. His curler was palmed away by Bobby Olejnik but Odubajo was on hand to tap in the rebound after 72 minutes.
Olejnik then kept his side in it with a couple of decent saves to deny Dean Cox.
O's boss Slade felt the London Road playing surface and the blustery conditions hampered his side.
"It was a bit of a bobbly pitch and the wind was difficult - my hair was blowing everywhere!" joked Slade, who is bald. "But it wasn't easy for both teams.
"I feel we created enough chances to probably go and score two or three goals, and that's the only slightly disappointing aspect. But as long as we are creating chances that's the main thing as I feel we will put some of them away on our pitch.
"To be honest, I was disappointed to be 1-0 down. The manner of the goal was particularly disappointing because he was clearly offside.
"But my players deserve credit for the way they came back, especially in the second half. They never give up and kept going forward. We created the better chances and thankfully we took one."