Hey Craig,
When you are creating the custom field for your drop down, select 'String' as the "Type".
I'm assuming, based off your choices listed, that this is going to be a field that's populated by a flow action (not an actual field on a form). If that's the case, just set up your smart campaigns to insert the appropriate option into that field.
If you were to use the field in a form, that's where you would add the options for a lead to select.
Hope this helps!