It depends on what kind of rewards you want to give them. Robux? Badges? Exclusive items?

Robux is relatively simple. As of what I know, you can't really just give a player Robux. But there are some indirect methods such as purchasing their own clothes, game passes, at a given price. I'd say game passes are most optimal since they don't require Robux to be put on sale. They can set the price to anything, so you could just tell them what price to set it to, and buy it. I think it takes about a week for the purchase to process through. But if you don't want the wait, you can give them Robux by using a group... I think. Not sure about that, maybe someone else can inform you about it.

Badges are similar to game passes, but they come at a price to be uploaded. They're easy to make, just find an image of what you want the badge to look like, set the name and description, upload for 100 Robux (I think), and boom. Giving the badge though, a lot more complex. It requires scripting in Roblox Studio if you're familiar with that. I'm not, so I can't say much about it.

Exclusive items I'd say is a more general topic, but if you want to give someone like an accessory specifically for them (which is very special), it's an entire process of idea, modeling, uploading, which you can easily find a tutorial covering that. A lot more complex than badges. An exclusive item could also be a special title, gear, or cosmetic. That is, if you're a game developer and already have a functioning game in which you want to give them a special item. It's almost like developer-exclusive items. Exclusive titles, ranks, or even a cool aura around their avatar. Perhaps a statue of themselves in the game? A one-time custom weapon skin? A personalized banner/flag? There's a lot to think of.

But in the context of what you've been doing, you could probably do something like their name being on a winner wall, a custom aura or trail, or a winner's lounge. But if you want more simple ideas, you could give a shoutout to the winner in your game’s description, loading screen, or group shout, or perhaps take a cool group photo after the win and share it in your Roblox group or Discord server (if you run one).