For this, you have to go to your Settings and scroll down to the "Forms You Are Filing" section:
You can read more about which forms you should be filing by referring to this article:
Forms You Are Filing
Now, to indicate that you're opting in to 8%, you then (1) select the appropriate Taxpayer Type. You also (2) select the "8% Gross Receipt Tax" from the Income Tax Rate Type here.
And that's it - you're done! Your forms will now reflect that you've chosen 8%.
Note that when you're setting up your profile for the first time, you also indicate these fields during set-up.