how to make values appear dynamically when adding an entry in a form?

how to make values appear dynamically when adding an entry in a form?

 I have a form where I have to add city names.

I first have to select the country from a drop down list which gets values from another form.

I want the form to work this way:

1) you select the county

2) then all the cities already added appear, just as a reference for the user

3) then u type in the name of the new city and save it.

should be simple, but i can't figure it out. I manage to get all the existing city names (as a stringlist), but how to display them on the form dynamically?

thank you,

Daniel