@typeOU Why would he be motivated anyway? If he doesnt go someone else will, its not like the title of hero is mandated by deities (except it is). And even if it is, nothing says he is the only option. Besides, if he is the only hero then why risk his own life to adventure and fight the demon lords, when he could just instead pick a territory to keep safe, easily besting any monsters that appear nearby. Or dont. Nothing stops him from leaving it all up to the armies of humans, the demon lord werent described as anyone capable of fielding armies enough to eradicate humanity. At most it sounds like someone antagonistic that will bring their era into a great turmoil until slain by a hero (possibly immortal otherwise), but ultimately incapable of being a true threat.
All that said, kidnapping someone to make him do it is stupid. He just needs enough power to deal with whatever villages guard her and to open the lock, which is far far less than what is needed to even reach the demon lords castle. Odds are that if he feels sufficiently betrayed the entire village would get wiped (although I doubt that, people arent that vindictive of places they grew up in). If they truly want to go the kidnapping route, it is better to just tell him that it is until he reached a specified level of competence. Ex. his first kill, or maybe a specified adventurer rank. Something low to make him bite the apple and do it without incurring too much ill-will and risk, simply to get his past the hurdle that is the fear of an entirely powerless civilian.
But better to just honey-trap him. I bet that if they ask the mage to "do it for the sake of the world" (for plot reasons she is obviously in love with him, so it serves as a great excuse for her to cinch the deal, anyway) and then leave on the journey with him until pregnant, she would. But maybe this plan requires too much meta-knowledge on tropes.