Switch from Classic to Polaris form.

I have customize my Polaris form, added a tab and write some JS on load of the form. 

When I run the form it always open in the classic view of Polaris form. If I remove that JS it runs fine in Polaris view. 

The form without JS is like this:

Form with JS is like this:

How to customize the form that the display remains the same????

Thanks!!!!!