How to have a dropdown/picker that allows user to input a value when it doesn't exist

You can do this with a custom picker: https://docs.appian.com/suite/help/24.2/Custom_Picker_Component.html

In the suggest function when a match isn't found for the value being passed as the filter you'll need to append that as a new value to the array of items used to search and return that array back to the calling interface. You'll need to do something similar with the identifiers. And then last of all, to ensure that the updated list is persisted you'll need to ensure the new values are written to wherever they are sourced from so that they're available for the next time you visit this interface,

You can do this with a custom picker: https://docs.appian.com/suite/help/24.2/Custom_Picker_Component.html

In the suggest function when a match isn't found for the value being passed as the filter you'll need to append that as a new value to the array of items used to search and return that array back to the calling interface. You'll need to do something similar with the identifiers. And then last of all, to ensure that the updated list is persisted you'll need to ensure the new values are written to wherever they are sourced from so that they're available for the next time you visit this interface,