Dropdown list in MsgBox (Macro Ms Project)

Dear All,
I am writing this post because I have no clue on how static/dynamic drop down list need to be configured in MS project Macros using VBA

I currently have a macro in MS Project that as per the input of the user concatenates the following information:
Country Code: Input from user
Product Code: input from user
Task code: Information store in mpp

Currently, Country code and product Code are manually entered by end user, but I would like the user to pick it from a predefined list.
How should this be configured? How can I store/manage the values available for both field?

Thanks in advance for your help

Francesc