The 26-year-old's wonder strike at Deepdale salvaged a 1-1 draw for the hosts of the opening encounter, after Alex Revell had given the visitors a first-half lead.
But Preston will now have to triumph away from home to take their place alongside Leyton Orient or Peterborough United in the final.
The club's play-off record will be of some concern to manager Simon Grayson, with Preston having never won the end-of-season knockout in eight attempts, losing in the semi-finals five times, but goalkeeper Declan Rudd is confident Garner can fire his side to Wembley on Thursday.
"He does it in training all the time and it’s just great that he’s got that confidence back he had a few months ago," he told the club's official website. "If you don't try it, it doesn’t come off for you and he tried it and it was an unbelievable goal and he’s done that so many times for us this season.
"He's been great and where we would be without him I don’t know and hopefully he can do it again on Thursday."
Grayson is chasing his fourth promotion from English football's third tier in seven seasons, but comes up against a Rotherham side who lost just two of their last 22 regular-season games to charge to a fourth-place finish.
The other semi-final is also finely poised at 1-1 heading into the second leg at Brisbane Road, with Moses Adubajo having cancelled out Britt Assombalonga's earlier effort on Saturday.
Orient captain Nathan Clarke was involved in six play-off campaigns during his time at Huddersfield Town - successful twice - and hopes to pass on some of his experience ahead of the return fixture.
"I've tried to talk with lads that have not been involved in these games before so they know what to expect," he told the BBC. "Personally, I think it's brilliant, exciting and great to be involved in.
"Personally, I'd be very proud to lead the team out for a final at Wembley. Obviously you think about it, but we're absolutely focussed on the semi."
Orient have not featured in the play-offs since their promotion from League Two to League One in 2006, while Peterborough are looking to bounce straight back to the Championship.
