Currently, there is no means to search by Username/Unique ID when adding participants to an event. We understand the need for this sort of feature and have created an Idea on our Community page to this effect. Please navigate to https://community.blackbaud.com and log in or create an account. Once logged in, you may navigate to the Idea page below in order to comment and vote on the Idea:

https://microedge.ideas.aha.io/ideas/ANGEL-I-12

Remember, the more votes an Idea receives, the more traction it will build with our Developers, so please encourage your colleagues to vote on the Idea as well!