Hello

You can refer to the post here where the same question is asked

• https://www.outsystems.com/forums/discussion/21969/mobile-bottom-bar-always-highlight-on-default-screen/
• https://www.outsystems.com/forums/discussion/66613/how-do-you-add-an-active-class-for-a-menu/

adding more to them
You need to define the index based on your pages, on which page your bottom bar lies 

When you open your screen and check the bottom bar the input is available which you have to pass

Make sure you have used bottom bar item widgets
https://success.outsystems.com/documentation/11/developing_an_application/design_ui/patterns/using_mobile_and_reactive_patterns/navigation/bottom_bar_item/

Thanks
 Tousif Khan