Adding admin bar/toolbar to non WordPress page

I found a similar topic here, but it seems it was closed before it was answered.

I have wordpress acting as the front page/ public side of my site, then I have a sub-directory like I would like to use as the member side(non wp), but I want to have it integrated into wp with the admin bar at the top.

If I can’t get the native wp admin/tool bar to show up, is there a way I can simulate this with my own?

I also want to control access to this site using the Membership plugin, so being able to use wp shortcode would be nice aswell.

File structure:

wp in root/main
member side in root/main/app