For this, you have to go to your Company Profile 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
You then select the appropriate Alphanumeric Tax Code (ATC) that corresponds to the 8% Tax Rate:
The ATC's that correspond to 8% are the following:
II015 - Business Income - 8% IT Rate
II016 - Mixed Income - 8% IT Rate
II017 - Income from Profession - 8% IT Rate
For more help in determining the ATC to select, you can read this article.
Note that when you're setting up your company/profile for the first time, you also indicate your preference during the set-up.