I did not recommended to remove that code from child theme’s functions.php, i just asked that if you removed or not.
To add #mg-inbox you need to override box-message.php (instruction in https://www.shamimsplugins.com/docs/front-end-pm-pro/customization-front-end-pm-pro/change-templates/) and add full url including #mg-inbox in action in line 14 and 20 ( eg. currently action="" will be action="http://example.com/inbox#mg-inbox" )