There are a number of ways you can make this work.
I would recommend using Modifier to change the window and then using GP Power Tools to add the business logic (because VBA is End of Life).
The next question is.... Is the list of valid values hardcoded or is it read from a table (and therefore easily changed by an administrator).
Methods:
1) Validate the Text Field so it only allows specific values.
2) Validate the Text Field and add a Lookup which shows the list of specific values from a table.
3) Change to a drop down list, with hard coded values stored in the local field definition.
4) Change to a drop down list, with dynamic values populated by code.
5) Change to a drop down list, with dynamics values populated from a table.
The options are limited only by your imagination.
Let me know if you want this changed as a demo, it would take no more than 30 mins for the most complex approach.
Regards
David