The event filter takes a single argument which must be a quoted string. It is true if the argument string is a substring of the Event field of the game.
    event "Open"

    event "1.p"
    not event "@"

The earliest correct (tested by the not event "@" above, using the conventions of the Heijden database) 1st prize study is from 1881.

The event can also be searched for from the command line:

    cql -event 1.p foo.cql