As a Roblox player, it is currently hard to donate to a game with your own custom amount, unlike developer products, that let you choose the amount of robux to donate set by the developer. Why not have an option in the place page to set your own custom donation?
Here’s a video example I edited down below.
For the slider maybe add in the amount they are donating, as a counter as of how far they are moving the slider so they know the amount they are donating.
Basically max amount is all the robux you have, and the minimum amount, is the smallest amount they want to donate. Maybe add in a radio button as an option if they want to donate that amount of robux monthly, and if they want to stop there’s an option to stop the monthly donating anytime.