Custom Action

I created a custom function called Sign_Up. This function will appear will appear whenever I click on an event on the calendar report. When clicked on the Sign_Up button, it will bring the user to the Events_Sign_Up page for users to register for an event. How can I program such that when the user is brought to the Event_Sign_Up page, the Event_field_name is automatically selected based on the event the user clicked on at the calendar report?