Create my own encounters

I am creating some fight encounters to simulate leveling out in the open world. So not quite a mythic+, and also no tanks or healers. So i wanted to know where do i find that menu to make those?

You can get to the theorycraft wiki from the simulator page:

Click there then click Theorycraft Wiki at the top. On the page you go to, choose a spec and the version (“Live” is the one you want to use).

Then press the NEW button at the top-middle of the page, and you can create a Boss script.

Note that the boss script editor can be a little challenging to use – if you have questions let us know.