United will face Newcastle in the showpiece on February 26 after they beat Nottingham Forest 2-0 in the semi-final second leg at Old Trafford to seal a 5-0 aggregate win.
The foundations were laid with a 3-0 triumph at the City Ground in last week's first leg and quickfire goals in the second half from Anthony Martial and Fred rubber-stamped their trip to the capital.
It is their first domestic final since 2018, when they lost the FA Cup, and puts them one win away from ending a six-year trophy drought, going back to the 2017 Europa League success under Jose Mourinho.
Ten Hag knows it is not job done, saying: "It's good to reach the final, but it's not about reaching the final, it's about winning it that's going to be tough, a great opponent.
"Now the focus is on the Premier League, in two days it will continue. It has to give the motivation in the first place, we want to bring that cup to the fans.
"But in this moment I'm not looking ahead, we're focusing on other things, a big week is coming up, we have Crystal Palace and then two games against Leeds and they are going to be great games, derby games that we are looking forward to."
Another positive for United was the return of Jadon Sancho, who made his first United appearance since October after a lengthy absence.
Sancho was "not in the right status or fitness state" having done an individual fitness programme in the Netherlands rather than travel to the squad's winter camp to Spain.
He returned to huge cheers from the Old Trafford crowd and Ten Hag is looking forward to getting the 22-year-old back to his best.
"Most important players (have to) enjoy football, which gives energy, that gives them the motivation to act and to perform," the Dutchman added.
"I think it was great, the reception from the fans to him, and also that will give him even more a push to continue in the way he's now acting, he's just started again, the restart he's back at Carrington for two weeks, we see that smile and hopefully he can keep that and contribute to the team.
"He already showed his quality this season for instance, the game against Liverpool, a great goal, scored against Leicester, he had some really good games, Spurs, he already showed his great capabilities.
"I hope when you are available all the time and play often together with the players we have now in the midfield and front line, our attacking play will improve and create more chances, then it's a process players enjoy more and better motivated to work together."
Forest boss Steve Cooper knew his side blew any chance they had by losing last week's first leg, but was proud of how far the club have come.
They had chances to make a game of it in the first half but Brennan Johnson and Emmanuel Dennis both missed and they went out with a whimper.
"We are overall disappointed we haven't got through because when you get this far you want to go one step further," he said.
"The damage was done last week and I couldn't say that before because it was a defeatist message. If we score the clear-cut chances in the first half, or even one of them, then it is a different game. United over the two legs have been far better than us.
"We shouldn't lose sight of the fact we have got to a semi-final and how many clubs would have loved to have been on the stage we have been on in the last couple of games? The first time in 30 years.
"This time last season we were 10th in the Championship, so for us to try and compete in the Premier League and get to semi-finals tells us that the club is on an upward projection."
There was a flashpoint in the second half when Johnson clashed with Alejandro Garnacho after the United winger appeared to say something inflammatory, but Cooper would not be drawn on the matter.
"Yeah, I don't want to comment on that," he said.