How to update `TextField` text with custom formatter prefix

I would like to create a custom Formatter to TextField. Here I have done some workaround but it's not working as expected.
We should show text in the formate of USD$ 200.00 USD$ prefix should show in the text field.

struct ContentView: View {
    @State var amount = ""
    var body: some View {
        VStack {
            TextField("USD$ 0.00",
                      value: $amount,
                      formatter: DollorFormatter())
                .keyboardType(.namePhonePad)
        }
        .padding(.horizontal)
    }
}

class DollorFormatter: Formatter {
    var text = ""
    override func string(for obj: Any?) -> String? {
        return text
    }

    override func getObjectValue(_ obj: AutoreleasingUnsafeMutablePointer<AnyObject?>?, for string: String, errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool {
        text = String(format: "USD$ %.2f", string.toDouble)
        print(text)
        obj?.pointee = text as AnyObject
        return false
    }
}

extension String {
    var toDouble: Double {
        return Double(self) ?? 0.0
    }