Although the various display frames of the MainUI are called pages, you shouldn’t really confuse them with standalone web pages. MainUI is really an app, and just like many of your other apps, it starts on the same launch point which lets you then navigate to other aspects of the app.
@hmerk just posted the same tutorial I was going to point out, as one option, but really, an even easier option is to work with the MainUI app and not circumvent it. Before you go through all these hacks, think about how to just use the overview page to be that main starting page that you’ve got.
It is a little trickier in your case, because you’ve created your own tabbed page which isn’t fully compatible with the overview page, but there’s actually nothing special about the tabbar on tab pages except for some formatting. With a little bit of rearrangement you can build your own tabbar like series of links that will then navigate to their own pages or simply hide/show different section of a single page. For an excellent example of this right now, check out the work that’s being done here: