Help me, I’m dumb

I’ve spent the last 4 hours trying to figure out this thing.
Basically I need to create (dynamically would be great!) a dropdown where when an user selects a city it get the corresponding listing of articles with the matching TV.
e.g. I select "new York" and I get all the documents listed with TV=New York
I actually have 2 test pages where the TV app_localita is set to "Treviso" and "Venezia"
Now...I’ve tried following some docs/tutorials i’ve found around the forum but nevertheless I got lost.
What I have atm is a chunk named CercaLocalita containing:
<form name="Localita" action="index.php?id=9" method="POST">
<select name="localita">
<option value="Treviso">Treviso</option>
<option value="Venezia">Venezia</option>
<option value="Padova">Padova</option>
</select>
<input name="Submit" type="submit" value="Search" />
</form>
Then a Search page with ID=9 containing:
[[!Ditto? &tpl=`site_ditto_app` &startID=`6` &filter=`app_localita,@EVAL return $_POST['app_localita'];,1 &noResults=`Nessun risultato.`!]]
And finally the following code in the homepage:
That’s it... the only result I get is that I am indeed redirected to the search page (id 9) but it always output "no documents found". The weird thing is that I even specified a custom error message (&noResults=`Nessun risultato.`). Last thing I’m using Modx Revolution.
help! I’m stuck!
Thanks for any suggestion