Go into Reports, Exports and Stats, and create a new report. Select the variables you want the report to contain, and write a report filter to include only those records you want. In this case, you might include a filter such as where a particular form is completed, and only those records for which that form is completed will appear in the report.
It isn't clear from your question what 'event' refers to. If it is a REDCap longitudinal event, then you might want to rely on, say, the completion of the last form in that event. If the event is [event_1_arm_1] and the last instrument on that event is [final_survey], then you could rely on the value of [event_1_arm_1][final_survey_complete]. If they have completed the 'final survey', then this value will be 2. If they haven't, then it will be not 2.
Thus, including a report filter such as [event_1_arm_1][final_survey_complete] = 2
will exclude from your report all records that have not completed event 1.
One you have a report that returns the correct records and your desired columns, you can export it in exactly the same way as you would export the 'all data' report.