Form -> Select List reload after entry field change

I am looking how to refresh a select list after having updated an entry field (like a numerical zip code for example).
example: I am typing the zip code and than I want to pick one of the city name linked to this code.

I found several examples but always talking about linked select lists.
I think I must have a look on AJAX events but I cannot find how to make my list updated

Thanks in advance for your help

Christian

Just found out one method for the upate [TABLE=“cellpadding: 0, cellspacing: 0”]
[TR]
[TD=“class: td1”] sc_redir( your_form, recordid= );
[/TD]
[/TR]
[/TABLE]
It is working for the refresh process but it is coming back on the previous version of the data (the record has not been saved).
If someone nows how to force the save process …