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.

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