It's not a feature in the game. Pokemon Dark Cry is a fan-made game, and it's likely not included because:
* Different mechanics: Fan-made games often have different mechanics and gameplay elements than official Pokemon games.
* No need for Strength: Dark Cry might not have features that require the Strength HM, like pushing boulders.
* Alternatives: The game might have other ways to achieve similar effects, such as using a specific move, an item, or a special character ability.
If you're looking for a game with the Strength HM, you'll need to stick to official Pokemon games.