Please note: this is not Partner Provider Codes, which are specified per provider. If you rather want to populate provider codes, you can use this article.
Navigate to Office Settings > Partners
Filter or search for your relevant partner, and click on the pencil to edit this partner:
Scroll down and expand More Actions to reveal the Partner Code field:
Type in the adviser's employee/partner number/code and click on Save.
You will now be able to include Partner Codes to your reporting: