Add drop down to Field

We would like to create a new field and have a field behavior tab enabled. There is a "assistant" field that we do not use which I have renamed. How can I add a drop down to that field?

Thank you for asking. You should use one of the predefined system Field1-5 for this. Rename them as needed and edit the list of dropdown option. Unlike the assistant field, that is meant to hold a name of someone, and thus does not have a dropdown, all these fields have their own fully customizable dropdown.

Hope this helps!