You'll be redirected to Roblox's gift cards page. Tap on Redeem, located at the very top of the page. Enter the empty Code field with your Roblox gift card's code, then tap Redeem to finish.

Related: Roblox Game Codes Remember to use Roblox script executors responsibly, as many of the scripts can give players incredibly unfair advantages in user-made games.

Disclaimer: Roblox scripts can get you banned in the game and also on the platform. So please use them at your own risk. Also, it is best to use scripts on new accounts rather than your regular ones.