Fundraising event - can I sell tickets AND collect donations?
one way you could do this is to create one field in your priceset for the ticket (probably making it required to ensure that they actually purchase tickets), and then another field (not required) that's a text field with a value set to 1 (which will multiply by the amount they enter), e.g., https://www.screencast.com/t/Db1nHAWfA (You can also set the financial type to something besides Event Fee for the additional contribution, if desired.)

Lesley