Chris Morrow struck 13 minutes from time to give Crusaders victory over holders Ballymena in Wednesday's Co Antrim Shield semi-final at Seaview.
The hosts created the better chances in a poor first half with Sean Cleary heading wide before Colin Coates failed to find the target from a corner.
The Crues pressed in the second half and substitute Morrow won it as his deflected shot hit the United net.
Stephen Baxter's side will take on Linfield in the 3 December decider.
Cleary was first to go close after 20 minutes from a free-kick before Coates squandered a chance 13 minutes later.
Crusaders dominated possession in a lacklustre opening 45 minutes but neither side looked dangerous.
Morrow came on for the second half and Crusaders were further galvanised by the introduction of forward Jordan Owens and winger Paul Heatley.
The Crues made a deserved breakthrough after 77 minutes, with Owens providing the knock-down and Morrow firing home thanks to decisive deflection.
The result stretches Crusaders' unbeaten run to 15 games - Ballymena were the last side to defeat the north Belfast men.
Linfield progressed to the final thanks to an impressive 4-1 victory against league champions Cliftonville on Tuesday night.