How do I blind specify value into drop down list?

I created a list which contain column type is Choice (menu choose from) — Display choices by Drop-Down Menu ex. IM 2.12, IM 1.021, IM 6.012 and allow user ‘Fill-in’ choices; user can freely to ‘Specify your own value’.

  • If user have others data that is not provided in the drop-down so they can fill-in there value on “Specify your own value” ex. IM 5.55 then next time the drop-down list MUST have user’s filled data such IM 2.12, IM 1.021, IM 6.012, IM 5.55

How can I achieve?