There is no method for you to change title bar behaviour of a particular app unless the developer has provided that functionality. Ask the developer of Roblox if it provides the functionality you want. Since the macOS version of Roblox is just a simple port of the iOS/iPadOS app (doesn't run on my Intel iMac), I rather doubt it. Apple's developer documentation is here https://developer.apple.com/documentation/swiftui/windowstyle/hiddentitlebar.