Set it to a Pop-up menu, with Allow other values, and have the value list based on From Field and the field you are using.
as brucewilson advised, switch to a value list based on a table. But using the same table that you are using for data entry as the source of your value list can sometimes be less than ideal. So an alternative approach is to set up a separate table as the source of your values and then don't use "other" as an option for your value list. Add a popover button for "Other" that opens to a global text field. A save button inside the popover can run a script that both:
Adds a new record with the given value to the value table and transfers the value to the field you've formatted with the value list.
This can look and function much like "other" but allow you to maintain the values in a separate table so that "other" adds them to the list.
Awesome. Thanks, everyone!