SetInputScope

public static class Extensions {

    public static void SetInputScope(this TextBox tb, InputScopeNameValue inputScopeNameValue) {
        tb.InputScope = new System.Windows.Input.InputScope() {
            Names = { new InputScopeName() { NameValue = inputScopeNameValue } }
        };
    }
}

textBoxAmount.SetInputScope(InputScopeNameValue.Number);