APM Webtop SSL-VPN pop up customization

Hello all,

I want to customize the pop up window used for webtop SSL-VPN and cannot found how.

I am able to customize Webtop, Login & Logout page. Edge client too. But I found nothing about this pop up.

I am using modern policy but this seems to use somehow the classic one as it has those diagonal stripes image background in header.

Any ideas?