Longman Dictionary of Contemporary English
door prize
noun
EXAMPLES FROM CORPUS
▪ A reconditioned 1950 model tractor will be given as a door prize.
▪ The girls swarmed around him like he was a door prize, their looks curious and teasing.
Wiktionary
door prize
alt. A gift given to people who attend a meeting, party, event, etc., especially one which is given on a random basis to one or some of the attendees. n. A gift given to people who attend a meeting, party, event, etc., especially one which is given on a random basis to one or some of the attendees.
WordNet
door prize
n. tickets are passed out at the entrance to a dance or party or other social function and a prize is awarded to the holder of the winning ticket
Wikipedia
Door prize