How do I set recurring appt for the 15th weekday of every month
 

I'm trying to set up a recurring appt based on the weekday number within the
month. For instance, the 15th or 20th weekday of the month (not to be
confused with the 15th or 20th of each month). I have been able to do this
up to the 4th weekday of the month (which has worked well) but the drop-down
list does not allow anything further out than that.