Here's how to add designations to your Donation Page:
Go to your "Donation Page" Settings in the platform, from the main menu:
Navigate to the 2nd step of content wizard (i.e. "Details"):
Scroll down to the end of the page, and find the Designations section:
From here you can add as many designations as you wish, and require that your donors make a selection as part of the donation process.