Hi @Shakthi Prakash,
As per my knowledge, it is impossible in SharePoint to allow users to dynamically add columns to an event list by adding custom web field. I would suggest you post a thread in this feedback portal: https://feedbackportal.microsoft.com/feedback/forum/06735c62-321c-ec11-b6e7-0022481f8472. SharePoint product team actively monitors questions there and will take your idea into consideration.
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