For an event registration form, a ticket option must be selected in order to make the additional donation. To allow users to only make an additional donation, you would need to enable a $0.00 event price option. This would allow the user to select the $0.00 price option, and then enter their desired additional donation amount.

How to create a free price option on an event in Blackbaud NetCommunity

Alternatively, you can create a link on the event registration page to a separate donation form page.