Hi Ahan,
I'm Amr, an independent advisor.
You can use a Microsoft gift card to purchase Roblox Premium. To use it, you need to add the gift card balance to your Microsoft account first.
Once the balance is added, you should be able to see the option to "Pay with Microsoft account balance" when making a purchase in the Roblox desktop app.
If you're still not seeing the option, make sure that you are signed in to the correct Microsoft account, and try restarting the app.
Remember that your Microsoft account needs to be linked to your Roblox account.
I hope this is helpful. If you require any additional assistance, don't hesitate to respond to this.
This is a user-to-user support forum. We're users just like you helping other users to find solutions to their problems.
Best Regard
Amr