The option to update/set field for UDF does not currently exist in Redtail. You can set a field with the trigger being "Activity Added", but the options of the field to update is limited to:
Activity Type
Activity Category
Activity Importance
Activity Priority
So does not include a UDF field.

Also, we would want the trigger to be Activity Category marked as completed - not added, so the UDF field would be updated once Activity was completed. Otherwise we would have to remember to delete, if Activity is cancelled. Currently a UDF as Review date field for each year. We add a new UDF filed for the current year to each Review Contact each year. The Review date field in Redtail is not a real / accurate date - but a suggested next review date. Would want automation to be able to Update UDF field with Activity Date when Activity with Activity Category Review was marked completed.