Navigate to the "Agencies" tab in the left toolbar and click into the agency you want to add an Advertiser to.

Select "Add Advertiser". Clicking this button will open a modal where you can enter the Advertiser's information.

Enter Advertiser Name

Set Competitive Categories and Create

All of the IAB's Categories and Sub-Categories are included in the modal. You can select as many as make sense for each Advertiser, but at least one category must be selected before creating. The categories are used for competitive separation - Advertisers with the same category associated won't run in the same podcast/episode. This is especially important if you are participating in the Spotify Audience Network.