Update Page Layout for New Event Tab on Opportunity

Hello!

I am trying to update the "New Event" Page Layout that I have placed on the Opportunity Page and remove the Category field. This is a Lightning Action, but I am unable to find where I can change this layout. I am using the Standard button for New Event and there are no Record Types and only 1 Page Layout on the Event object.  Any suggestions on where to look? 

#Lightning Experience Customization