Are you on the classic experience?
The new experience allows for drop downs to kind of simulate the typeahead experience.
However, typeahead fields are used to reference objects in Workfront. Often with multiple key, values.
If there's some reason you want to hack a list of options into a typeahead field, I think you could still use the approach of using a more hidden object in Workfront (like Company) and apply a filter to the field using text mode.
You could add a custom form with a field to "tag" the companies you want to populate the typeahead field.
This would work, but is definitely hack-y.