I am trying to write a trigger that will create an activity when an opportunity reaches a certain stage.  I want the activity to be assigned to the owner of the opportunity and to be related to the opportunity, but when I try to assign values to the appropriate fields I get 'Field is not writeable' errors.  Here is the code with these fields commented out:

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trigger CreateActivityAfterClosedWon on Opportunity (after insert, after update) {
 
    List<Task> NewTasks = new List<Task>();
    for (Opportunity opp: Trigger.new){
        if (opp.StageName == '100 - Closed Won - Booked'){
                NewTasks.add(new Task(
                    ActivityDate = Date.today(),
                    //What = opp.Id,
                    //Owner = opp.Owner,
                    Status = 'Closed',
                    Activity_Type__c = 'Project Won',
                    Priority = 'Normal',
                    Subject = opp.Name));
        }
    }

    insert NewTasks;
}

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With these fields commented out, the trigger runs in the sand box and creates activities, but the activities are not related to any object and are assigned to me (because the code ran under my account).

Can anybody suggest a solution?

Thanks & Regards,

Jeff