The key question isn't so much about the form; it's about the database that the form is based on. If you create the database through OpenOffice.org you will be forced to create a key, so that's not a problem.
To modify a form, just open the database that it's based on, then click the Forms icon at the left side, and when you see the form name, right-click on it and choose Edit.
The form will open. Right-click on any component and choose Control or Form to modify it. You can also use the Form Controls toolbar.
Choose View > Toolbars > Form Controls, when you have the form open, if you don't see the toolbar.
To make the list you're talking about, I'm not enough of a database expert to do the table relations planning. However, you could set up a view between the two tables, the main and the secondary. See my article on views.
Then, modify the form and choose to replace the City field with a list box.
Right-click on the control and in the Data tab, specify the field to bring in the data from.
You could also just create a field with those cities in them. Create a list field using the Form Controls toolbar, and don't connect it to a table. Just right-click on it and choose Control. Then, in the window that appears, look for the List Entries field and just type your entries. See my article on modifying forms; there's a lot of detail in there about creating a hardcoded list field.
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