This example assumes you have two forms set up: one for the event and one for reservations. The event form will include a field for the number of available openings. The reservation form will include a Dynamic field to list all the events. This function will drop the number of available openings for an event when it is selected.

The following code can be used to conditionally change the default value in a field, depending on a parameter in the URL. Change 25, 26, and 27 to your field IDs. Replace 'pass_entry' (in two places) with the parameter name in your URL.