If your visitors are receiving an error message that says...
"INVALID_FIELD: No such column 'sbx_NumberOfTickets__c' on entity 'Lead'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names."
...it's likely because the Soapbox Events QuickStart for Salesforce package has not been installed. This package includes custom fields that are used by Soapbox Engage's Events app to record information about registered attendees.
To learn more about installing the Soapbox Events QuickStart for Salesforce package, walk through the Soapbox Engage Orientation guide's first step.